Mixing Zone Workshop 1 & 2 Question and Answer Battery 1 08 April 2011 & 25 May 2011

What does ‘proportionate’ mean in the definition of mixing zone?

It means that the extent of an acceptable mixing zone is linked with the characteristics of the receiving waters. For example, a mixing zone of a certain size may be clearly acceptable in the context of receiving waters being an open coastal water body but clearly unacceptable in the context of a river or small lake. There are no hard and fast rules but the mixing zone guidance assists in deciding what to consider in specific circumstances.

The tier 1 assessment proposes indicative values for the allowable increase in concentration after complete mixing. How were these figures derived?

The Working Group wished to provide some guideline values to use as a conservative screening mechanism to rule out from further consideration discharges that are of small impact on overall water quality. This is essentially not a scientific exercise, but these values were derived by expert judgement from the working group members as a pragmatic approach to limiting the requirement for more detailed study to those discharges of greatest significance. Effectively the criteria used in Tier 1 were based on the definition of a mixing zone as described in the Guidance document

Can the Fischer Equations really describe the complexity of mixing in real waters?

The Fischer equations provide a convenient way of estimating the mixing of a discharge into the receiving waters in a range of circumstances (eg the flow does not vary too much in space or time, the bank and bed are fairly uniform [see e.g. Rutherford 1994 ref in Guidance]). Within the precautionary approach of tier2, they can provide a useful way of informing decisions on mixing zone identification. However, there are other models available which could be used at tier 2 (eg CORMIX). In some circumstances it may not be possible to use such models in a tier 2 approach (eg discharge on a river bend just downstream from a major bridge with piers) in which cases other approaches would be more suitable. The tiered approach allows and encourages different levels of approximation of the real world and levels of effort for regulator and discharger commensurate with the nature of the decision to be made.

Since emission, discharge and loss of PHS such as cadmium will cease in 2020 why bother with mixing zones at all?

Since some PS and PHS, are naturally occurring their emission will never cease completely as noted by recital 4 in 2008/105/EC. For many substances there are ‘stores’ which will persist for many years. The obligation is to adopt measures which aim to progressively reduce/cease emission (2000/60 art16 (1). It is questionable that the timetable suggested of 2020 is achievable for such substances. 2008/105/EC did not introduce additional measures. It introduced an obligation for the Commission to provide a progress report in 2018. Mixing Zone Workshop 1 & 2 Question and Answer Battery 2 08 April 2011 & 25 May 2011

How do we handle situations in which data quality (eg analysis LoQ, sampling uncertainty etc) is insufficient to quantify emissions reliably or even whether < or > EQS?

There may be other ways of characterising the emission. For example knowledge of the processes influencing the composition of the effluent in question may allow better characterisation of the effluent than simply sampling and analysing the ‘final effluent’ (eg if there are streams which dominate the load which can be measured at lower flow and higher concentration which then mix with lower concentration process streams prior to final discharge).

Is the EQS-AA or the EQS-MAC more important?

Either or both can be relevant depending on the circumstances of the case. Initial inspection of the available data (discharge flows and concentrations, receiving water flows and concentrations) can be helpful in identifying the relative importance. Care needs to be taken when considering compliance with a MAC (as a 100%ile) and a statistical approach can be useful (see CMEP Plenary paper from Bratislava meeting in November 2010).

Do we use total or dissolved concentrations for metals?

The approach to metals should be considered carefully. In the early tiers it may be appropriate to consider total emissions (ie sum of dissolved + particulate phase load in the discharge) but treat this emission as if it were dispersing in the dissolved phase in the receiving waters for purposes of modelling and comparison with an EQS set for dissolved phase substance. This will be a precautionary approach. However, where there is confidence in the partitioning applicable in the situation of interest it may be appropriate to make an assumption regarding the proportion of the emitted load which will occur in the dissolved phase in the receiving waters. It is the partitioning in the dissolved phase in the receiving waters which is important, and this may be different to the partitioning in the effluent (which may be at different pH etc to the receiving waters). Many different approaches are possible to partitioning in a tier 3 approach. The guidance encourages the use of the simplest assumption consistent with providing a result with sufficient confidence.

Why bother with mixing zones for point sources when the real WrFD problems are with diffuse sources?

The priorities for regulatory activity are a matter for case specific consideration. In some cases a diffuse source could be considered as a number of point sources and the Guidance and Discharge Test software may assist in regulatory decision making. However, the terms of reference for the Guidance are specifically for point sources (2008/105/EC Art 4.1 designation of mixing zone adjacent to points of discharge).

Does the Guidance apply to specific pollutants (Annex VIII) as well as PS and PHS? Mixing Zone Workshop 1 & 2 Question and Answer Battery 3 08 April 2011 & 25 May 2011

Although the requirement for development of the Guidance stems from 2008/105 which deals exclusively with PS and PHS, the drafting group were aware that the principles were no different for other determinands of interest and the Guidance was developed with this in mind. It is hoped that Member States choosing to use mixing zones will find the Guidance useful for a wider range of determinands.

Does the discharge test software only represent single port discharges? Does the discharge test software assume a discharge perpendicular to the flow or can it apply for angled discharges?

Yes, but models which include more complex outfalls (such as diffusers) often deal with them via defining an equivalent single port or slot. The discharge test assumes a discharge perpendicular to the flow of the water body. There is no option available in the discharge test to deal with angled discharges.

The DELTARES software allows different locations for a discharge – bank or centre of river, surface or bottom – how much difference does this make?

This can have some influence on the mixing zone particularly where there may be plume buoyancy effects or interaction of the plume with the bankside.

The location of the discharge can have influence on the calculated mixing. In this perspective both the horizontal position and de vertical position is of importance.

Horizontal position: In the discharge test two options are available: discharge at the bank of the water body or a discharge in the middle of the water body. In addition, a higher streaming velocity in the middle of the water body will improve mixing compared to a bankside discharge, where currents are generally lower. (current velocities near the discharge point are some of the input parameters).

Vertical position: In case of stratification or a situation where buoyancy effects occur (resulting in a rising or sinking plume), the vertical position of the discharge can affect the mixing behaviour. In case of stratification (density jump), the density together with de vertical position of the discharge can affect the mixing behaviour. Both density of the discharge compared to the density of the water body at discharge depth and the vertical position of the discharge can influence the vertical mixing length and thus the calculated mixing factor.

What if there little data is available, are there standard parameters defined in the discharge test?

Some default parameters are included in the model, but expert judgement may be applied that is appropriate for case specific parameters

Does the Discharge Test software deal with multiple discharges? Mixing Zone Workshop 1 & 2 Question and Answer Battery 4 08 April 2011 & 25 May 2011

The software can be used to provide indicative results for two plumes interacting by, for example, taking the background concentration at the downstream discharge as defined by the ‘ambient’ concentration in the receiving waters + the increase in concentration at the downstream discharge location resulting from the upstream discharge. This will be a precautionary approach since concentrations resulting from the upstream discharge will continue to decline downstream. Care would need to be taken when the discharges were not neutrally buoyant or located at different banks of the water body.

If the upstream discharge is sufficiently far away that mixing across the section can be considered complete, incrementing the background concentration will be appropriate.

How do we handle cases where there is a sensitive receptor located on the opposite bank to the discharge?

The principles of the Guidance apply to this situation in that it requires the potential impact on this receptor to be considered.

The Discharge Test software has default lengths of interest for a mixing zone alongshore extent based on precautionary extents as set out in the Guidance (relating to river width, water body length or an arbitrary length of 1000m). However, the software allows the user to prescribe other lengths to suit individual case circumstances such as a sensitive receptor being located closer than the downstream distance provided by the default precautionary extents. The principles underlying the discharge test software allow evaluation of concentration profiles at the opposite bank. However, for ease of use, in its current configuration, the software provides user output only for concentrations along the bank at which a bankside discharge is located, these in most circumstances being higher than those occurring on the opposite bank. The web based application presents the mixing factor along the centreline of the plume, i.e. the highest concentration in the plume at a certain distance from the source. There are other means of estimating concentrations on the opposite bank. The Fischer equations may be used. Some models such as CORMIX allow user inference of concentrations directly (through the assumption at each phase of the calculation of a lateral profile of the plume (eg ‘top hat’ or Gaussian).

It should be noted that use of the Discharge Test software in tier 2 is to be regarded as part of the ‘sieve’ and in some cases it may be necessary to consider alternative approaches in tier 2 or move to tier 3.

What are the key technical assumptions which underlie the Discharge Test software evaluation of concentration distributions?

Implementation of the jet/plume modelling of the initial mixing induced by the outfall arrangements is precautionary in some respects since once the initial jet/plume phase is over, no further mixing in the vertical is permitted once the plume reaches the water surface or bottom. Due to the fact that banks of a water body generally can be seen as valuable parts of the water body in an ecological sense, mixing is calculated at the bank of the water body from the plume centreline (maximum concentration). Considering the mixing Mixing Zone Workshop 1 & 2 Question and Answer Battery 5 08 April 2011 & 25 May 2011 zone as a whole this can be seen as a worst case approach. More details can be found in the Technical background document.

Can the software remember the case between sessions?

At present once defined and run the session parameters are recorded in output documentation which can field and, if necessary, retyped into the web tool. A policy decision in tool development was made not to make use of ‘cookies’. However, it is recognised that a regulator dealing with a large number of cases may value having session files stored on the local machine which define the case and which can be easily re-read by the software (eg for audit purposes and to facilitate future work). Deltares will note this as a useful development possibility. This would also facilitate user support via a support desk.

Can the software deal with other pollutants such as Nitrate, Phosphate & suspended solids?

The software can be used for any pollutant which can be regarded as a conservative substance in the case of interest. There may be cases for which this assumption is appropriate for nitrogen and phosphate but not in general. The modelling of suspended solids requires care to consider the particle size distributions, flocculation and the potential for settling or erosion and this is beyond the scope of the Discharge test software. Such considerations would lie within tier 3 modelling/assessment.

.What happens if the EQS is already exceeded?

This is a policy decision rather than a mixing zone calculation per se. It may be that it indicates a need to revisit discharges upstream, to reduce emissions and create headroom for new discharge. If of sufficient benefit, or reasons of disproportionate cost, or infeasibility, it may be that exemptions are appropriate for that water body. Even if the EQS is exceeded at some locations in the water body, the nature and extent of the revised EQS exceedence resulting from the proposed discharge would need to be considered in the light of the MZ Guidance and the CIS Guidance on monitoring before determining whether or not the water body as a whole meets its chemical status objective (and hence whether or not the mixing zone associated with the proposed discharge can be accepted). In this respect there may be differences between the way the determination is made for PS/PHS and for other determinands (see 2008/105/ EC Annex 1 Part B 1 which prescribes the principle for PS/PHS EQS- AA for representative monitoring points). Whether a monitoring point is representative or not may depend on the distribution of discharges and mixing zones within that water body.

Do you always assume the maximum discharge flow that the permit allows?

That may be a reasonable starting point for a worst case assessment. However if better information is available that can be used to represent actual flows it may be appropriate to use those. This may be considered a tier 3 approach, but in fact this could be used using the simple discharge test, rather than necessarily requiring more complex assessment. Mixing Zone Workshop 1 & 2 Question and Answer Battery 6 08 April 2011 & 25 May 2011

Does the discharge test apply to lakes with minimal or no discharge?

In such circumstances the lake would be predicted to eventually attain concentrations equivalent to the discharge as there is no dilution flow. This would be a case where more complex modelling may be needed to account for losses through decay reactions, volatilisation or partitioning with sediments - these processes are not included in the discharge test, but may be considered in Tier 3 modelling.

Does the mixing zone encompass a number of contaminants of concern that may be present in a discharge?

Mixing zones may be calculated for all chemicals of concern (CoC) and for both AA EQS and MAC EQS. However, unless background concentrations are an overriding factor, the CoC with the highest ratio of concentration : EQS will define the largest mixing zone and hence will often be the most critical to consider.

Who would be expected to pay for more complex modelling at tier 3 and 4?

That is essentially a decision for Member States. In practice where a Regulator chose not to make a decision to allow a mixing zone on the basis of tier 1 or 2 assessment the applicant would be faced with either:  modifying the operation so as to produce a discharge which could be allowed a mixing zone through a tier 1 or 2 assessment or  carrying out a tier 3 / 4 assessment. This would often be at the expense of the applicant. However, it should be borne in mind that although tier 3 and 4 assessments can be time consuming and costly, depending on what models and data on the water body is already available, the wider societal costs involved in decisions to modify a process leading to a discharge or denying a discharge (so as to close an operation) may be very considerably greater. WrFD encourages the use of measures which are proportionate.