Guidance

Infiltration systems: groundwater risk assessments

How to assess the risks to groundwater for treated effluent discharges.

You must submit a groundwater risk assessment as part of your environmental permit application if you want to discharge treated sewage effluent into the ground through an infiltration system.

This guide does not apply to discharges to the land surface (for example, unlined reed beds or grass plots).

This guide includes the specific risk assessment requirements for trade effluent, domestic and non-domestic treated sewage effluent discharges to the ground through constructed infiltration systems. It’s mainly for larger, complex discharges, or discharges in sensitive locations.

Your infiltration system or drainage field should meet the definition in BS6297:2007 (+ A1:2008). It should be a series of infiltration pipes, placed in single trenches or one large bed, used to discharge effluent so it percolates into the disposal area.

You must not discharge raw sewage directly either to:

  • the ground
  • sub-surface drainage systems (those below the surface)

You must have an appropriate level of pre-treatment before discharge to an infiltration system – depending on the environmental risk your system may need to consist of:

  • primary treatment, such as a septic tank system
  • primary and secondary treatment, such as a biological treatment (for example, a package treatment plant)
  • primary, secondary and tertiary treatment (such as a reed bed)

You must not discharge sewage effluent to surface water without secondary treatment.

During the risk assessment you’ll need to decide whether the frequency and cumulative discharge volume will be a factor in risk and whether the discharge is acceptable. The outcome from your environmental risk assessment will help you decide on the most appropriate level of treatment for your proposed sewage effluent discharge.

Risk assessments: who carries it out

A risk assessment must be carried out before you can be granted a permit – this is known as ‘prior examination’ in the Environmental Permitting Regulations 2016.

A permit may also only be granted as long as you carry out an appropriate level of monitoring to assess the impacts on groundwater quality from the discharge. This is referred to in the Environmental Permitting Regulations 2016 as ‘requisite surveillance’.

The amount of monitoring will depend on the risks posed by the discharge and the consequences of the inputs of polluting substances into groundwater.

You need to provide the Environment Agency with all the information it needs to either review or carry out the risk assessment on your behalf. If you do not, it’s likely to lead to your application not being ‘duly made’ and the Environment Agency may reject your application.

When the Environment Agency will do the risk assessment for you

Check if the Environment Agency will carry out the risk assessment for you, otherwise you must carry it out yourself. Whether the Environment Agency will carry out the risk assessment for you is based on factors including the quantity and quality of your proposed discharge and the sensitivity of the groundwater where you plan to make the discharge.

If you have intermittent or seasonal discharges use the maximum daily discharge rate when applying the 2m3 or 15m3 per day thresholds to decide who will carry out the risk assessment.

In all cases you must provide the Environment Agency the information it needs to carry out the risk assessment on your behalf.

When you carry out the risk assessment yourself

If you carry out the risk assessment yourself you’ll need to provide the information that your risk assessment is based upon, along with your completed assessment, to the Environment Agency for it to review. You need to send this with the application form as part of the application process.

Existing permits

You must carry out a new risk assessment (following the tiered approach) if either of the following apply:

  • you’re changing (varying) an existing permit and the Environment Agency requires you to do so
  • the conditions on your permit say you must carry out a new risk assessment

You can use groundwater monitoring data to support your new risk assessment as appropriate.

How to apply for a permit

You can only apply for a permit to discharge to the ground if you can show that it’s not reasonable for you to connect to a mains sewer.

You can find out more on what the Environment Agency may see as reasonable in position statement G5 of the Environment Agency’s approach to groundwater protection.

You must assess the risks to show that your activity will not cause pollution or harm the environment. The Environment Agency will assess your application and if satisfied will grant you an environmental permit to discharge treated effluent to the ground under the Environmental Permitting Regulations 2016.

You must assess the system and ground to see if any hazardous substances or non-hazardous pollutants in your discharge will cause pollution. Hazardous substances and non-hazardous pollutants (with the exception of ammoniacal nitrogen, ammonium and suspended solids) are known as specific substances.

You should aim in your assessment to identify the appropriate location, design and operation of liquid effluent treatment and disposal to minimise the risk to groundwater.

You will also need to examine the following (‘prior examination’) as part of your application:

  • hydrogeological conditions
  • the purifying powers of the soil and subsoil
  • the risk of pollution and alteration of the quality of the groundwater.

If you’re granted a permit its conditions will require you to follow any necessary technical precautions to:

  • prevent the input of hazardous substances to groundwater
  • limit the input of non-hazardous pollutants to groundwater to make sure such inputs do not cause pollution of groundwater

The Environment Agency’s approach to groundwater protection: how it may affect you

The Environment Agency’s approach to groundwater protection sets out where certain activities may be restricted or where special rules apply. You should read section G on discharge of liquid effluents into the ground.

Sewage disposal: systems you must have

You must not discharge raw sewage directly to the ground, or to sub-surface drainage systems. You must have a liquid effluent disposal system that treats sewage effluent waste in these steps.

  1. The system collects and treats raw effluent – for example, septic tanks (primary treatment) and additional treatment through package treatment plants (secondary treatment). In some situations you’ll need a tertiary treatment, such as a reed bed.
  2. The system collects and distributes treated effluent to the infiltration system.
  3. The system discharges the treated effluent to the ground through the infiltration system or drainage field.

Sewage effluent discharges to infiltration systems mainly come from:

  • septic tank systems
  • package treatment plants
  • sewage treatment works

Septic tank systems consist of a subsurface infiltration system and connecting pipe work. They allow raw sewage to undergo settlement or flotation (physical separation) of the solids, storage and partial decomposition (anaerobic digestion) in the tank. The liquid is released through the inspection chamber and distribution box to the drainage field.

Package treatment plants are small-scale sewage treatment works and they treat effluent to a higher standard than septic tanks.

Non-sewage trade effluents (for example, from the treatment of industrial waste waters) can come from other treatment plants.

You must make sure that new and replacement sewage treatment plants and septic tanks follow British Standard (BS EN 12566). You may not be granted a permit if they do not meet this standard.

Cesspools, soakage pits and boreholes

Cesspools (also known as cesspits) or boreholes are not classed as infiltration systems. Using them creates a higher risk of groundwater pollution.

A cesspool should be a sealed unit with no discharge to the environment – you must empty it regularly and dispose of it correctly.

You should not normally discharge to soakage pits because they concentrate the discharge in one place and bypass the soil layers. This limits the potential for contaminants to be attenuated. The Environment Agency will not grant a permit for these discharges.

In line with the Environment Agency’s approach to groundwater protection you can only discharge effluent into boreholes if both of the following apply:

  • there’s no alternative
  • you have evidence of a large enough unsaturated zone (the part of the subsurface above the groundwater table) beneath the base of the borehole for effective contaminant attenuation

You’ll need to complete a detailed quantitative risk assessment before the Environment Agency will consider granting a permit for discharges to boreholes.

Infiltration systems: location and design requirements

Follow British Standard BS6297:2007 (+ A1:2008) for your infiltration system and drainage field design and installation.

You can get further guidance on drainage and waste disposal in Part H of the Building Regulations (2002 edition).

Infiltration system and drainage field locations

To minimise the risk to groundwater, you need to make sure that the size of the drainage field is appropriate to the:

  • rate of discharge
  • the infiltration capacity of the ground

For example, this means it follows the design and location specifications in British Standard BS6297:2007 (+ A1:2008). If your infiltration system does meet the standard then you need to carry out your risk assessment using the Environment Agency risk assessment tool (Annex J5).

You must make sure that there’s sufficient depth to the water table (at least 1.2m above the seasonally highest groundwater level) to make sure the effluent can attenuate. You should also consider how close receptors (like water supplies and surface watercourses) are.

The most important requirements to protect groundwater are that the discharge to a drainage field should not take place on land:

  • within 10m of the nearest watercourse
  • within 50m of a well, spring, borehole or other source of water intended for human consumption
  • that’s steeply sloping or waterlogged
  • where there’s less than 1.2m depth to water table below the invert of the drainage pipes
  • where percolation rates fall outside an upper and lower range of values

Domestic or trade effluent

Whether your discharge is classed as domestic sewage or trade effluent is one of the main considerations the Environment Agency makes when reviewing your risk assessment and deciding whether to grant your permit application.

You must classify your effluent as domestic or trade. The amount of monitoring you need to do for your risk assessment depends on the:

  • type and amount of effluent you’ll discharge
  • type of site where your effluent will infiltrate

You should not need to carry out a detailed investigation to determine if a discharge contains trade effluent – if the effluent is broadly domestic in nature then it’s classed as domestic sewage.

If a significant proportion of the effluent generated by a commercial enterprise is different from that found in a normal home it becomes a mixture of domestic sewage and trade effluent and is classed as trade. This will affect whether the Environment Agency will carry out the risk assessment for you.

Domestic sewage includes effluent from:

  • toilets
  • swimming pool waste
  • personal washing, showering and bathing
  • cooking at home for family and friends
  • household washing of clothes and bedding using domestic soaps and detergents
  • washing dishes and cooking equipment after using them on the premises

Some commercial activities can count as domestic, including:

  • commercial cooking – for sale directly to consumers who’ll eat either off or on your premises (like a restaurant, pub, fast food outlet or sandwich bar)
  • washing clothes or linen from activities or residents at a commercial site (like a camp site launderette)

However, if the effluent quality generated by your commercial activity is different from that found in a normal home it becomes a mixture of domestic sewage and trade effluent.

Trade includes effluent from:

  • commercial cooking – for sale off the site (like making ready meals or jams, making sandwiches for sale at petrol stations or canteens)
  • washing at commercial sites of clothes or linen from off the site, including bedding, tablecloths and towels (like a camp site launderette open to non-residents, a high street launderette or centralised laundry for a hotel chain)

The amount of monitoring you need to do for your risk assessment depends on the:

  • type and amount of effluent you’ll discharge
  • type of site where your effluent will infiltrate

Hospitals, vets, schools and universities

Hospitals, vets, schools and university effluents have domestic or trade aspects to their discharges depending on the type of activities on site.

For example, a small hospital may only discharge domestic sewage while a large, more complex hospital may have a range of non-domestic effluent.

You must class effluents as trade if your discharge includes animal or clinical wastes that you would not expect in domestic sewage.

Trade effluent

If your discharge contains trade effluent you’ll need to assess the likely quality of the trade effluent. You need to estimate how much trade effluent (minimum, maximum and average) you’ll discharge to the ground throughout the year, and consider whether this amount might change.

The Environment Agency may ask you for actual measurements (usually if you discharge over 50m3 per day). This may include constant flow monitoring subject to conditions in the permit. The Environment Agency may need you to measure actual flows of effluent via the permit to ‘back up’ any risk assessment estimates that may have been used originally.

Depending on recommendations from local Environment Agency teams, you may also need to send information to the Environment Agency on the treatment system and the substances it has been designed to treat. For example, biochemical oxygen demand (BOD) rather than pesticides or herbicide, which may be the main risk to groundwater.

You should review your analytical data to identify any substances that exceed the relevant environmental standards for the receptors at risk. If you have time series data you should create a graph to show how it changes.

Trade effluent’s chemical composition will depend on the:

  • activities that produce effluent
  • type of treatment process

Some trade effluents may also vary though the year.

This means you’ll need to show the trade effluent’s chemical composition by chemical analysis to determine whether hazardous substances and non-hazardous pollutants are present. For example, you can use minimum, average, maximum or 90th to 95th percentile concentrations.

Risk assessments: information needed

The level and detail of information you need to include in your risk assessment depends on several factors. These include:

  • the amount and type of effluent you discharge
  • the sensitivity of the environment you plan to discharge to
  • potential pathways to receptors at risk from your discharge

The level of information includes basic, extended and comprehensive information, each needing more detail than the previous level.

For sewage discharges:

  • up to 15m3 per day – basic information, although for discharges that do not comply with the exemption criteria or those that fail the Environment Agency’s in-house risk assessment, you may need to send an extended set of information
  • between 15 and 50m3 per day – extended or comprehensive information
  • over 50m3 per day – comprehensive information

For trade effluent discharges:

  • up to 25m3 per day – extended or comprehensive information
  • over 25m3 per day – comprehensive

For combined sewer overflows (CSO) and emergency overflows (EO), extended or comprehensive set of information may be needed.

Assess the quality of the treated effluent

You must consider the quality and composition of treated effluent in your proposed discharge. The chemical composition of effluent depends on:

  • the effluent source (whether it’s domestic sewage or trade)
  • type of treatment
  • operation and maintenance of the treatment system

You should consider either the concentration, or the range and variability of concentrations, of all the hazardous substances and non-hazardous pollutants you may release as a result of your discharge.

If you have a wide range of substances present you’ll need to identify representative substances or groups of substances.

Your risk assessment will then need to focus on both of the following:

  • whether or not there’s likely to be an input of hazardous substances to groundwater
  • whether any inputs of non-hazardous pollutants to groundwater will cause pollution

Substances of concern

The main substances of concern (substances you must consider in your risk assessment) for sewage effluent discharges are:

  • ammoniacal nitrogen (the ammonium ion NH4+), a non-hazardous pollutant
  • ammonia (NH3), also known as free ammonia or unionised ammonia
  • microbiological contaminants – these are not substances in the definition of groundwater activities in the Environmental Permitting Regulations 2016, but can contain harmful contaminants (such as pathogens like bacteria, viruses or protozoa)
  • phosphorus and phosphates, non-hazardous pollutants
  • other contaminants – including hazardous organic compounds and household chemicals, such as cadmium and mercury, which are hazardous substances

If you have a domestic septic tank or package treatment plant the discharge quality is likely to remain stable around these typical values and should not contain unusual substances. If so you should be able to use minimum reporting value (MRV) figures in your risk assessment.

You’ll need to analyse the effluent as part of your quantitative risk assessment if:

  • this is not appropriate for you (for example, you have trade substances from your activity that is not domestic in nature)
  • you’ll discharge over 50m3 per day

For both domestic and non-domestic systems you should try to prevent disposing of pharmaceuticals, pesticides, solvents, oils and greases.

Hazardous substances and non-hazardous pollutants

Hazardous substances

You’ll need to show that there’s no discernible concentration of hazardous substances, or exceed the minimum reporting value (MRV), either:

  • in the discharge
  • that can be attributed to discharge in the groundwater immediately down-gradient of the discharge zone

You can read about how to assess the discernibility of hazardous substances.

You’ll need to either monitor the discharge down-gradient of the discharge zone or, if it’s a new discharge, have a detailed predictive hydrogeological impact assessment.

If you have or expect to find discernible concentrations of hazardous substances in the groundwater immediately down-gradient of the discharge zone, you’ll only be granted a permit if all of the following apply:

  • concentrations will not lead to actual pollution or a significant risk of pollution in the future
  • there’ll be no progressive increase in the concentration of hazardous substances beyond the immediate discharge zone (there will be no statistically significant and sustained upward trend or significant increasing frequency of pollutant ‘spikes’)
  • you’ve taken all necessary and reasonable measures to avoid the input of hazardous substances to groundwater

Necessary and reasonable measures

As an operator you must take all necessary and reasonable measures to prevent hazardous substances from entering groundwater.

A reasonable measure is one where the necessary technical precautions to prevent inputs to groundwater are:

  • technically feasible
  • not disproportionately costly
  • within your control

Measures could include:

  • source control
  • altering the discharge mechanism (like improving the type of treatment system – for example, going from secondary to tertiary, or increasing the drainage field area or enhancing the design)
  • treating the discharge
  • intercepting or diverting contaminated groundwater
  • diverting the discharge to another disposal route

Any measures you take must not create more harm overall to the environment than the issue they mean to resolve (‘lead to net environment disbenefit’).

If there’s actual pollution, or a substantial risk of pollution you must take remedial measures – this is for you to propose to the Environment Agency. You cannot use a cost benefit assessment in deciding whether to take action in these cases. But you can consider the cost benefit in deciding which precautions are necessary.

Non-hazardous pollutants

You must limit the input of non-hazardous pollutants to avoid pollution, both with direct and indirect inputs to groundwater.

You must be able to prove to the Environment Agency that you will not cause pollution to groundwater. You must also make sure that any input of non-hazardous pollutants either:

  • avoids creating a deterioration in the status of a groundwater body, as shown in your local river basin management plans
  • does not lead to an environmentally significant and sustained upward trend (you cannot let pollutant concentrations increase over time) in the concentration of pollutants

You must also take all necessary and reasonable measures to limit the input of non-hazardous pollutants to groundwater to avoid pollution.

Discharge rate, frequency and duration

Your risk assessment must include details of the treatment system’s :

  • discharge rate (in m3 per day)
  • frequency of the discharge (constant or intermittent)
  • discharge duration

Most discharges have discharge rates that are fairly constant throughout the year. If your discharge rates vary during the year, you should include the range in your assessment. For example, your discharge is from a holiday cottage or hotel, or from a sewage treatment works with summer or dry weather flows.

You can estimate discharges based on the type of premises. The Environment Agency may ask you for actual measurements if your trade discharge is over 50m3 a day.

Detailed calculations

You can get a more detailed breakdown of calculations you need to use in your risk assessment and examples in Groundwater risk assessments for infiltration systems calculations and examples (PDF, 304KB, 7 pages) .

Estimate the discharge rate for sewage: domestic properties

Follow these steps to work out the total sewage output in m3 per day for single properties.

  1. Work out the P value for your home – this is the minimum recommended population load based on the number of bedrooms in each property plus 2 (for example, a 3 bedroom house has a P value of 3 + 2 = 5P).
  2. Multiply the P value by the current sewage generation rate per person per day – this is 150 litres per person per day in British Water’s code of practice – flows and loads 4 (for example 5 x 150 = 750).
  3. Divide this figure by 1,000 to get the rate in m3 per day (for example 750/1,000 = 0.75m3 per day).

Follow these steps to work out the total sewage output in m3 per day for groups of homes.

  1. Work out the P value for each home (for example, a 2 bedroom house has a P value of 2 + 2 = 4P, while a 5 bedroom house has a P value of 7).
  2. Add all the P values from each home together (for example 22P for a group of 2, 3, 4 and a 5 bedroom homes).
  3. If the total P for a group of houses is in 12 to 25 range or 26 to 50 range you can reduce to allow for the balancing effects of daily flows – multiply by 0.9 for P values between 12 and 25, or multiply by 0.8 for P values between 26 and 50 (for example P value of 22 x 0.9 = 19.8).
  4. Round up any fractions to a whole number (for example round up 19.8 to 20, or for a P value of 36 x 0.8 = 28.8 round up to 29).
  5. Multiply the P value by the current sewage generation rate per person per day – this is 150 litres per person per day in British Water’s code of practice – flows and loads 4 (for example 150 x 22) = 3,300).
  6. Divide this figure by 1,000 to get the rate in m3 per day (for example 3,300/1,000 = 3.30m3 per day).

If there are larger groups of houses you should estimate the P value using both the expected total load and the flow, considering both peak and total flow.

Estimate the discharge rate for sewage: non-domestic properties

To work out the total sewage output in m3 per day for non-domestic discharges you must do the following.

  1. Work out the current sewage generation rate per person per day – use British Water’s code of practice – flows and loads 4.
  2. Multiply this number by the number of persons at the business.

For example, for a hotel, follow these steps.

  1. The sewage generation rate for a 3 or 4 star hotel = 250 litres per person per day.
  2. Number of persons = 75.
  3. Discharge = 75 x 250 = 18 750 litres per day, or 18.75m3 per day.

For example, for 150 people at a static serviced caravan park, follow these steps.

  1. The sewage generation rate for a serviced caravan park = 150 litres per person per day.
  2. Number of persons = 150.
  3. Discharge = 150 x 150 = 22,500 litres per person per day, or 22.5m3 per day.

Percolation tests

You need to carry out a percolation test and use its results as this is an important part of your risk assessment.

You must carry out a percolation test to:

  • get the soil’s drainage characteristics
  • calculate the drainage field’s floor area

This lets you determine whether the rate of percolation through the soil is suitable for building an infiltration system that follows British Standards. Building a system that follows the standards reduces the groundwater pollution risk.

Carry out the test in line with British Standard BS6297:2007 (+A1:2008) and the Building Regulations (Part H 2015) to get a percolation value (Vp).

Carry out a percolation test: summary

Before the test you must dig a trial hole to find out the minimum depth to the water table (the standard is 1.5m below the base of your proposed trench).

If your trial hole suggests that attenuation may not happen you may need to dig deeper to the water table. Otherwise you may need to consider:

  • modifying your infiltration system
  • reducing the risk in other ways, such as increasing the level of treatment

This might be the case:

  • for larger discharges
  • where the effluent contains hazardous substances
  • where the underlying strata are characterised by rapid flow (for example, fissured aquifers)

You need to avoid carrying out the test in extreme weather conditions such as drought, frost and heavy rain. You then need to follow these steps.

  1. Excavate at least 2 holes 300mm square and 300mm deep below the proposed invert level (bottom of pipe) of the infiltration pipe. Space them evenly along the proposed line of the irrigation system.
  2. Fill each hole with at least 300mm of water and let it seep away overnight.
  3. Next day, refill each hole with at least 300mm of water. Then time how long (in seconds) it takes for the water to seep away from 75% full to 25% full (a total change in depth of 150mm).
  4. Divide this time by 150 to get the average time in seconds for the water to drop 1mm.
  5. Repeat steps 1 to 4 at least 3 times with at least 2 trial holes.
  6. Calculate the average figure for Vp by adding all the values from each test and dividing by the number of values used.

Results

Your Vp must be between 15 and 100 seconds per mm. If the soil is too permeable (the Vp is between 1 and 15 seconds per mm) the effluent will rapidly infiltrate, allowing little or no attenuation and you could pollute the groundwater.

If it’s not permeable enough (the Vp is above 100 seconds per mm) the effluent will not infiltrate well and you may get ponding (the unwanted pooling of water). This could lead to overland pollution problems and odours.

You’ll need to discuss alternative options with the Environment Agency if:

  • the Vp is between 1 and 15 seconds per mm
  • the Vp is greater than 100 seconds per mm

These alternative options could include:

  • if the soil’s too permeable – reducing the infiltration rate through the subsurface with engineered drainage mounds or sand filters
  • if the soil’s not permeable enough – you’ll need to use an alternative disposal method other than an infiltration system

Calculate the drainage field area

You’ll need the percolation value (Vp) to calculate the required drainage field floor area (A) of the drainage field in square metres (m2).

For domestic properties you’ll also need the number of people served (the maximum who could live in the house) by the treatment system. For:

  • septic tanks use this formula A = p x Vp x 0.25 - multiply the percolation value by the number of people (p) the tank serves by 0.25
  • package treatment plants and other liquid effluents with secondary treatment use this formula A = p x Vp x 0.2 - multiply the percolation value by the number of people the plant serves by 0.2

For example, if you have 12 people with a treatment system discharging to a drainage field with a percolation value of 20 seconds per mm, the floor area will be:

  • for septic tanks: A = 12 x 20 x 0.25 = 60m2
  • for a secondary treatment plant: A = 12 x 20 x 0.2 = 48m2

Floor area conversion

You should convert the floor area to a linear trench based on the width of the trench.

Drainage trench widths should be between 0.3m and 0.9m. Examples of conversions for different trench widths are given in British Standard 6297:2007+A1:2008.

For example, a 0.9m wide trench with a calculated floor area of 60m2 will give a linear trench length of 66 m.

Calculate the dry weather flow

Dry weather flow (DWF) is only needed for water company sewage treatment works that receive sewage and rainwater from combined sewers. If you have a private sewage treatment system you should avoid allowing rainwater into the plant.

DWF is the average daily flow to a sewage treatment works during a period without rain.

The formula is explained in detail in Groundwater risk assessments for infiltration systems calculations and examples (PDF, 304KB, 7 pages) . In summary, you can estimate the DWF by following these steps:

  1. Work out the total water consumption per day by multiplying the number of people served by the works by the daily water consumption (usually 150 litres per person per day, for example 4,000 people x 150 = 600,000.
  2. Work out the infiltration of groundwater into the sewer – this is usually 50% of the figure in step 1 (for example 0.5 x 600,000 = 300,000), but can be 0 if the sewer is above the groundwater table and so there’s no infiltration.
  3. Work out the trade effluent inflow (for example 160,000 litres per day).
  4. Add the figures in steps 1 to 3 together (for example 600,000 + 300,000 + 160,000 = 1,060,000 litres per day).
  5. Divide this figure by 1,000 to get the rate in m3 per day (for example 1,060,000/1,000 = 1,060m3 per day).

Calculate the infiltration rate: summary

You must calculate the infiltration rate (also known as the hydraulic loading rate).

How to calculate the infiltration rate is explained in detail in Groundwater risk assessments for infiltration systems calculations and examples (PDF, 304KB, 7 pages) . In summary, you can work out the filtration rate through the infiltration system in metres per day as follows:

  1. Calculate your discharge rate to the field (either estimated or measured), either by measuring or estimating it, in m3 per day.
  2. Calculate the area of the field in square metres.
  3. Divide your discharge rate by the area.

For example, for a septic tank serving 12 people (p) discharging to a drainage field within a Vp of 20 seconds per mm:

  1. Discharge rate to drainage field = 12 x 150 = 1,800 litres per day or 1.8m3 per day.
  2. Area of drainage field = 0.25 x p x Vp = 0.25 x 12 x 20 = 60m2.
  3. Infiltration rate through the infiltration system = 1.8/60 = 0.03 metres per day.

If the infiltration rate is greater than the maximum infiltration rate from your percolation tests, you must re-design your drainage field.

If your discharge includes surface water run-off, you may need to include storage in your design for peak flows and the infiltration rate should be set as the maximum infiltration rate.

Potential for mounding of the water table

If the infiltration rate is high, but the capacity of the saturated aquifer to carry this water away is limited, the discharge may lead to higher water levels and so reduce the thickness of the unsaturated zone.

This means the Environment Agency may need you to carry out an additional assessment for larger discharges. You may then need to modify the drainage field (for example, use of drainage mounds as described in BS 6297:2007 + (A1:2008).

You can estimate the extent of mounding of the water table by either:

  • calculating it
  • for an existing operation, measuring groundwater levels in boreholes next to the field

Risk assessment approach

When you have sufficient information and carried out your initial calculations you can start on your risk assessment. You should carry out your risk assessment in a tiered approach (stages) as laid out in the guide to groundwater permits. Each stage is more complex and requires more data and information than the stage before it:

  1. Tier 1: Risk screening.
  2. Tier 2: Generic quantitative risk assessment (GQRA).
  3. Tier 3: Detailed quantitative risk assessment (DQRA).

After risk screening you may identify that your proposed discharge is acceptable from a groundwater pollution risk perspective. If the Environment Agency agrees it’ll grant you a permit, providing all other aspects of your application are equally acceptable. If not then you’ll need a more detailed assessment (for example, taking it to the next stage – a generic quantitative risk assessment).

The level of information you need to include in your risk assessment depends on:

  • the discharge volumes
  • type of effluent you’re proposing to discharge
  • your site’s hydrogeological setting

For a quantitative risk assessment you’ll use calculations or numerical models to assess:

  • the impact of a discharge on groundwater
  • the significance of attenuation (the reduction in contamination concentration in groundwater over time)
  • the available dilution (only for non-hazardous pollutants)

You must make sure that only a competent person (such as an experienced hydrogeologist) creates any models you use. These models must be based on a strong conceptual model using hydrogeological expertise.

Before you carry out a risk assessment

Before you carry out a risk assessment you should read the guide to groundwater risk assessments understand terms, concepts and risks in groundwater permitting and what to do, including:

  • how to carry a groundwater risk assessment and what the stages are (the ‘tiered approach’)
  • what compliance points and values are, and how to set them
  • what risk screening is and when and how to carry it out
  • what a conceptual model is and how to develop one
  • what pathways, sources and receptors are and the source pathway receptor model for risk assessments
  • how to investigate your natural background levels for contaminants
  • understanding what groundwater and aquifers are and how your discharge may affect them
  • how to send the application, and what you need to send (including an electronic copy of your risk assessment model)

You should also check if you need to carry out any other risk assessments.

Develop a conceptual model

Before carrying out your risk assessment you must develop a conceptual model. Your conceptual model will form the basis for your risk assessment and you’ll need to refine it as your assessment progresses.

Details of how to do this are in the guide to groundwater risk assessments.

Basic information: what you must include

All risk assessments must include this basic level information.

You must state in your permit application the site details, including:

  • details of premises – domestic (single or multiple dwelling), industrial, sewage treatment works
  • where the discharge is – 12 figure grid reference and a location map at 1:10,000 scale
  • details of drainage field – plan area, depth, details of construction and drainage
  • number of persons associated with the discharge (if relevant, for example for housing or an office)

You must state how you’ll treat the effluent, including:

  • type of effluent – domestic sewage, trade effluent or both (combined)
  • details of any effluent pre-treatment – septic tank, package treatment plant, septic tank, reed bed or other ways

You must state what the site’s geology is, including:

You must include calculations and test results for discharge quality and loading, including:

  • rate of discharge (m3 per day) – measured or calculated based on number of persons and type of premises or estimated dry weather flow
  • the hydraulic loading rate (also known as the infiltration rate)
  • results of percolation or other infiltration tests
  • discharge composition – result of chemical tests or published data (for small sewage discharges)

You must state what the site’s environmental setting is, including:

  • groundwater environment – Principal Aquifer, Secondary A or B Aquifers, unproductive strata, it’s in a source protection zone (SPZ), water protection zone or safeguard zones (SgZ)
  • depth to the water table (unsaturated zone thickness) – include seasonal water level variation, evidence of water logging, details of any water seepage or standing water levels in trial pits
  • hydraulic conductivity of the aquifer – you can use literature values (for example the physical properties of major aquifers in England and Wales)

For how close the discharge is to surface watercourses you may also need to send information on:

  • stream flows
  • water quality
  • ecological status
  • what the water’s used for (for example, fish farm or as a source of potable water supply)

This will depend on:

  • what your first risk assessment outcomes produce
  • if your site is in a sensitive location

You may also need to get further details relating to nearby water abstraction points, including the abstraction type and what you use it for.

You must also include any historical and environmental information, including:

  • flood risk and if the site is in a floodplain
  • details of historical discharges to the drainage field
  • any sites of special scientific interest or species action plans

Extended information: what you must include

You need to get extended information if you have larger and more complex discharges so you can carry out an appropriate level tiered risk assessment.

If the Environment Agency asks you to send extended information you must also include all of the basic information and:

  • historical land use – industrial site, contaminated land or other
  • details of operation and maintenance – monitoring of discharge rates, effluent quality, liquid levels in distribution chamber
  • flow mechanism – fissure or intergranular flow
  • direction of groundwater flow – groundwater monitoring boreholes
  • hydraulic gradient – if you have groundwater monitoring boreholes that let you measure the groundwater level (boreholes that can be dipped) you can use them to calculate the hydraulic gradient
  • effective porosity of the aquifer – you can use literature values
  • mixing depth – from borehole logs, groundwater quality sampling or theoretical calculations
  • soil or subsoil characteristics (where relevant) to 3m below base of drainage field and borehole logs and trial pits
  • hydraulic conductivity of the aquifer – site specific data

Comprehensive information: what you must include

The Environment Agency may ask you to provide comprehensive information depending on the risk your discharge represents and the sensitivity of receptors. You’ll need to provide with your risk assessment extended information if you discharge:

  • between 15 and 50m3 of sewage effluent a day
  • 25m3 a day or less of trade effluent
  • you have combined sewage overflows or emergency overflows

If the Environment Agency asks you to provide comprehensive information you must also include all of the basic and extended information and:

  • soil and rock properties – lithology (description of rocks), moisture content, fraction of organic carbon content and other factors for retardation
  • groundwater quality, up-gradient and down-gradient of discharge field from groundwater monitoring boreholes

Carry out a risk screening assessment (tier 1)

For treated domestic sewage effluent discharges up to 15m3 per day (outside an SPZ1) the Environment Agency will carry out a risk screening assessment for you. It will base it on the information you send with your application. The Environment Agency will then determine whether you will need to carry out any further assessment or provide further information.

For all other discharges you’ll need to include a risk screening section in your own risk assessment. From an initial risk screening of the site, your discharge can be shown to be acceptable based on one or more of the following:

  • the discharge has concentrations of non-hazardous pollutants less than the relevant environmental standard or natural background level applicable to the receiving groundwater
  • the presence of unproductive drift or unproductive bedrock strata (there are no aquifers beneath or near the activity) and remoteness from surface waters means that risk to any identified groundwater dependent receptor is very low
  • the volume (hydraulic loading rate) of the discharge is very small, so only minimal dilution in underlying groundwater will be needed to avoid pollution by non-hazardous pollutants
  • the discharge has concentrations of hazardous substances sufficiently close to the relevant MRV or the natural background level in groundwater (whichever’s the higher level) for you to assess that attenuation processes in the unsaturated zone or immediate dilution at the water table will prevent the input of hazardous substances

When the discharge has concentrations of hazardous substances sufficiently close to the relevant MRV – this value is usually at detection level or agreed minimum practical analytical value. You need to assess at the qualitative level that the attenuation processes in the unsaturated zone, or immediate dilution at the water table, will prevent the input of hazardous substances.

The basic information required for a screening assessment includes:

  • the size of discharge
  • the results of percolation tests
  • depth to the water table
  • geology (soils and strata descriptions and thicknesses from logs from excavations)
  • details of, and closeness to, receptors

Carry out a generic quantitative risk assessment (tier 2)

If following tier 1 risk screening you need to carry out a GQRA you must understand the source, pathway and receptor terms with respect to your discharge and its environmental settings.

You should use conservative assumptions for input values for dilution, unretarded and retarded travel time, and attenuation factor.

GQRA is appropriate for activities when both of the following apply:

  • you can well-define the source
  • the known properties of the soil are easily capable of reducing risks to underlying groundwater to ‘low’, even if there are uncertainties in the thicknesses and properties of the underlying strata

Find out more on what’s needed in a GQRA. You can see the examples in Groundwater risk assessments for infiltration systems calculations and examples (PDF, 304KB, 7 pages) but you should use the J5 infiltration worksheet for your calculations.

You should only use this or other risk assessment tools when you have a good conceptual model and you believe that the worksheet calculations are relevant to it.

If you choose not to use the infiltration worksheet then you’ll need to use an equivalent model. The model will need to use the basic equations (from Groundwater risk assessments for infiltration systems calculations and examples (PDF, 304KB, 7 pages) ) for determining the impact of a discharge on groundwater quality and for determining the discharge limit values.

Carry out a detailed quantitative risk assessment (tier 3)

You’ll need to carry out a DQRA if you’ve identified a potential risk in the GQRA based on relatively simple calculations and conservative assumptions.

A DQRA needs more detailed site specific information and you’ll need to support this with investigations. You’ll typically use a more complex probabilistic modelling approach to assess the impact of uncertainties in input data.

You may also need to carry out a DQRA if the quantity and quality of the discharge may change significantly through time (this may be the case for trade effluent discharges).

Modelling and using tools

You can use the Environment Agency spreadsheet tool (J5 infiltration worksheet) to assess discharges to infiltration systems.

The infiltration worksheet and other risk assessment tools should only be used by suitably experienced hydrogeologists or those experienced with the approach to groundwater risk assessments.

You need to assess whether using this spreadsheet tool is appropriate for your site (for example, it is not suitable for highly fractured and fissure flow as it was developed for matrix flow). If it is not, or you choose not to use the J5 infiltration worksheet, then you must use an alternative numerical model and explain why you’ve used it.

More complex risk assessment models

If you need to use more complex DQRA risk assessment models with more detailed site-specific information, you’ll typically have to use a probabilistic modelling approach. This is to assess the impact of uncertainties in input data.

You’ll need realistic input values that assume distributions (for example normal or log normal) of values within the range for models (like the Environment Agency’s ConSim model).

The ConSim model includes an option to model discharges to soakaways (a pit where you pipe waste water so that it drains slowly into the surrounding soil). You can treat the soakaway as an infiltration system by using the area and infiltration rate from percolation tests.

Set compliance points for risk assessments

You must set compliance points for discharges to the ground if you carry out a DQRA or GQRA.

You must not breach compliance values when carrying out your risk assessment modelling. If you do you’ll either need to increase your waste treatment process, or update and improve the design of your drainage system.

You can see where to set compliance points in the main guide to groundwater risk assessments.

You can also see the illustration with guidance in Where to set compliance points for groundwater risk assessments for infiltration systems (PDF, 87.1KB, 1 page)

Site investigations

You may have to carry out a site investigation as part of your risk assessment to get the:

  • depth of the groundwater
  • hydraulic gradient and flow regime
  • properties (like moisture content, effective porosity, hydraulic conductivity, presence of fractures or fissures)
  • background quality
  • site specific details, like the effects of earlier discharges

You’ll also need to:

The Environment Agency uses the data from investigation monitoring and your risk assessment submission to assess whether it’ll grant you a permit. This differs from monitoring to check ongoing compliance with a permit known as ‘requisite surveillance’ (the required monitoring of groundwater as part of the permitting process).

Substances to model

Your risk assessment must concentrate on the hazardous substances and, as necessary, non-hazardous pollutants in the effluent discharge that are most likely to have an effect on groundwater or related sensitive receptors.

You must identify substances to model based on:

  • hazardous substances in high concentrations compared to their MRVs and background concentrations in groundwater
  • non-hazardous pollutants in high concentrations compared to their appropriate environmental substances

From these you should consider substances that during the movement of the discharge down through the subsurface soils, rocks and strata are:

  • least likely to be attenuated (and could be used as a marker substance in monitoring)
  • attenuated by different mechanisms (for example the metal cadmium being attenuated by sorption or precipitation, or ammonium being attenuated due to transformation to nitrate)

See the substances of concern for what you should look for.

Hazardous substances

You must set a compliance point for groundwater at its point of entry into the saturated zone immediately below the infiltration field. The saturation zone is the area of an aquifer below the water table where all the pores and fractures are saturated with water at a greater pressure than atmospheric pressure.

If you need to monitor boreholes to confirm your risk assessment outcomes you must set the compliance point as near as possible to the point of entry into the saturated zone.

You need to allow only for the instant dilution that happens as the discharge comes into contact with the groundwater. But not before attenuation in the saturated zone, or dilution by groundwater flow below or outside the mixing zone, happens.

Non-hazardous pollutants

For non-hazardous pollutants you can set as compliance points:

  • an existing water use (like an abstraction borehole, spring, wetland, stream or river)
  • a point between this receptor and the discharge along the contaminant pathway

If the groundwater resource (rather than defined receptors) is at risk you need to consider the environmental sensitivity of the aquifer. For example, how important it is locally for a source of drinking water.

The receptor in this case would be a theoretical abstraction borehole at a point no more than:

Larger discharges

For larger discharges you should include the main substances of concern most likely to be found in sewage effluent in your assessment, and:

  • orthophosphate, especially if the receptor is surface water and its falls under the Habitats Directive designation, for example a Special Protection Area or Special Area of Conservation (SPA or SAC)
  • metals (depending on the results of effluent analysis)
  • hazardous substances (depending on the results of effluent analysis)
  • hydrocarbons (depending on the results of effluent analysis)
  • boron (depending on the results of effluent analysis)
  • other substances, like pharmaceuticals or chlorinated hydrocarbons

For large sewage effluent discharges (more than 50m3 a day) you’ll need to carry out a trade effluent chemical analysis.

The need for chemical analysis for smaller sewage effluent discharges will be site specific.

Dilution and attenuation processes to consider in your assessment

Several important processes are likely to reduce the impact on groundwater from liquid effluent discharges to a drainage field. You need to consider these processes as necessary in your assessment:

  • sorption – the reaction between the aquifer matrix and solute that removes solutes from groundwater and reduces the apparent rate of solute movement
  • retardation – the reduction in the apparent rate of solute movement
  • biodegradation – the reduction in contaminant concentration as microbes degrade contaminants, and is highly dependent on the geochemical environment
  • abiotic degradation – the chemical transformations that degrade contaminants, and is dependent on contaminant properties and geochemical environment
  • volatilisation – the process that removes contaminants from groundwater and transfers them to soil gas
  • dispersion – reduces contaminant concentrations due to groundwater moving at different rates or through different pathways in the aquifer
  • diffusion – contaminant moving from area of higher concentration to areas of lower concentration, reducing contaminant concentrations

Attenuation and groundwater flow

If you have a large discharge or infiltration system in a sensitive area (for example an SPZ or safeguard zone (SgZ) established for nitrate) you’ll need to include supporting information on the properties of the unsaturated zone, including:

  • thickness and lithology of soils, sub-soil and rock
  • moisture content
  • soil bulk density
  • fraction of organic carbon

You also need to include the groundwater flow in the aquifer below the drainage field (flow direction, hydraulic gradient and aquifer properties).

If you identify fissures or fractures

If you identify fissures or fractures in your preliminary risk screening assessment you need to take this into account in your risk assessment modelling. The infiltration worksheet’s calculations for dilution and attenuation factors assume that flow is through the matrix of the soil or rock and that there’s no rapid bypass flow (it’s ‘intergranular’).

Most rocks have some fissuring. You can partly account for this by adjusting input values such as porosity and permeability to make it comparable to an intergranular flow system.

If fissure flow is significant enough that you cannot compare reasonably, or if the rock is karstic and rapid bypass flow is more likely, you’ll need to make your assessment more conservative. Do this by using simple dilution and assuming high permeability.

Attenuation is likely to be very limited in fast flow karstic situations. You’ll need to use field evidence, such as groundwater tracer testing and flow measurement, and you may need to use a more sophisticated modelling approach.

Highly fissured or karstic limestone systems

For highly fissured or karstic limestone systems (for example those identified on local scale geology maps) your models need to be conservative and allow for dilution only.

You may need to do more detailed investigations to confirm that the discharge infiltration area is not over a major fissure or karstic feature. Your percolation tests should show if there are rapidly draining strata.

For dual porosity systems, such as the Chalk, then you can assume an equivalent effective porosity (diffusion between fissure and pore water).

You need to design the system so that the discharge rate does not exceed the hydraulic capacity of unsaturated zone, leading to rapid bypass.

You’ll need to send an electronic copy with your application of the model to the Environment Agency to check your approach and relevant site specific data. If you use the infiltration worksheet or ConSim then send a copy of that. If you’ve used your own model you’ll need to send that and your workings on how you put it together.

The Environment Agency may ask you to show that karst features (such as sink holes, swallow holes or dolines) or other preferential pathways are not present in the vicinity of the drainage field.

Monitor the discharge activity

You may have already carried out some monitoring as part of your application. If you’re granted a permit you may need to continue to monitor your activity to make sure that you stay within the limits of the risk assessment. Your permit conditions will state how often and what determinands (for example, what chemicals are important) you’ll need to monitor for.

You may need to do one or more of the following:

  • operational checks and records (for example of sampling, maintenance and inspection checks)
  • discharge rate and effluent quality measurements
  • groundwater levels and groundwater quality in boreholes located around the infiltration system measurements
  • water quality in related receptors measurements

Under the ‘charges for discharges’ scheme the Environment Agency will normally carry out essential monitoring of the effluent and groundwater (if any existing boreholes are present). But if you have an Operator Self Monitoring agreement, or if your permit requires you to do so, you’ll do some of the monitoring.

You’ll need to provide the necessary monitoring infrastructure and access arrangements whether it’s you on the Environment Agency that carries out the monitoring.

Your permit will state if you need to carry out the discharge flow rate measurements.

Effluent monitoring

For treated sewage effluent your permit will state what parameters you may need to monitor. Typically the substances you need to monitor will be ammonium and total inorganic nitrogen. For larger discharge, you should set out the recommendations for monitoring of effluent discharge rate and quality, including:

  • the frequency of measurement
  • the parameters you’ll measure

You may need to carry out a total nitrogen monitoring on a site specific basis. This depends on the risk the effluent discharges represents to the water environment, particularly in sensitive locations such as SgZs where nitrate concentrations in the groundwater are of concern.

Additional monitoring

The Environment Agency may ask you to monitor your site as part of your investigation process if effluent at the site:

  • is already being discharged
  • effluent has been discharged in the past

This can involve:

  • down-gradient and up-gradient monitoring
  • monitoring of the effluent itself

Reporting: what you must send with your application

You must give a full and clear description of activities in your risk assessment. The Environment Agency may reject your application if it’s not confident about the models you’ve used or the information you’ve provided. Similarly, if the Environment Agency is carrying out the risk assessment on your behalf it will need the information necessary to carry out the assessment for you.

You must send details of your effluent treatment and the site, including:

  • details of the type and source of the effluent (for example, domestic sewage)
  • details of the discharge rate, frequency and duration
  • details of the treatment process
  • your hydraulic loading rate (also known as the infiltration rate)
  • information on the depth to water table and the soil description

You must also send details of your site and your equipment, including:

  • a description of how you’ll control the quality of the construction or engineering, for example building regulations certificates
  • details of how you’ll use and maintain the system to make sure the system works as designed (like de-sludging septic tanks, servicing package sewage treatment plants)

You’ll also need to send details of the infiltration system or drainage field including:

  • a map showing location and layout of the drainage field
  • the design of drainage system (including plans and cross sections)
  • the area of drainage field and how worked this out
  • the results of infiltration or percolation tests

You should not locate drainage fields in areas known to suffer from flooding as this can lead to pollution and health issues.

See how and where to submit your risk assessment.

Models and data

When you’re carrying out your own risk assessment, you must send an electronic copy of the risk assessment models you use with your application. As part of the permit determination process the Environment Agency will check your risk analysis. You should send any relevant site data with it.

Essential monitoring recommendations

You should send recommendations for essential groundwater monitoring. You must refer to your conceptual model when designing the monitoring system.

Published 1 February 2016
Last updated 3 April 2018 + show all updates
  1. All hazardous substances and non-hazardous pollutants (with the exception of ammoniacal nitrogen, ammonium and suspended solids) are known as specific substances. If your discharge includes specific substances, your risk assessment will need to include a specific substances assessment.
  2. First published.