How to Use the Best Management Practices (BMPs)

Introduction

Unlike a sewage treatment works or an industrial plant, the pollution from a farm does not discharge from a single pipe (or point source). Diffuse pollution can be a result of a number of small sources distributed across the farm, which cumulatively have a significant negative effect on water quality in rivers and streams running through the landscape.

Much of this type of pollution, called diffuse pollution, can be controlled by introducing a series of control measures, specific to each type of source, distributed across the farm. These measures are usually referred to as Best Management Practices.

Pollution can be considered to originate from a source in the landscape, be moved from the source via series of pathways and eventually reach a receptor loch, pond, stream or river. A BMP, or more often a suite of BMPs, introduced as close to the source as possible can reduce pollution risk.

Following this concept, the BMPs are split into six different applications :

those most likely to be applied to reduce pollution from the steading area
those which can be applied in arable fields
those which are applicable to livestock
those which can be applied to woodland areas
those that intercept pollution on the edge of the water body (riparian)
approaches that require a consideration of farm-wide effects and require some sort of general planning or management approach

The BMP data sheets are designed for use in conjunction with a Diffuse Pollution Audit and Siting and Suitability guidance.

The BMP data sheets follow a common format:

Name	Descriptive name for the BMP
Background	Putting the BMP into context and describing in general terms how it acts.
Description	How is the BMP is applied?
Type	Six types are considered: steading, arable (in-field), livestock, woodland, riparian (bankside) and planning and management measures.
Intervention Mode	For pollutants that are lost by surface run-off, five factors influence the rate of transfer of the pollutant to a water body: 1) the size of the source; 2) the rate of surface water run-off; 3) the ease of dislodgement of particles at the soil surface; 4) the velocity of water runoff; 5) the “connectivity” between the source and the water body, i.e. is there a direct path between the source and the water body. The connectivity can be considered as being influenced by two components, the proximity of the source to the water body and any features in the landscape which intercept and retain the pollutant on the land (Figure 1). Figure 1 Process which need to be considered in determining the most appropriate BMP Most BMPs reduce the input of pollutants to a river by altering one of the other factors. The size of the source is usually controlled by reducing the inputs; the rate of water run-off is usually achieved by either increasing the percolation rate into the soil or by detaining the water for a period of time to reduce peak flows. Dislodging particles from the soil surface is the first process that must occur for particles to be transported by water. Since particles are most easily dislodged from bare ground most BMPs to control dislodgement aim to maintain ground cover. The velocity of run-off water is generally reduced by reducing the length of slope. The connectivity between a source and a river can be decreased by BMPs which create an interception barrier that retains the pollutant on the land. The BMP would always be to reduce the input of a pollutant. If this is not possible economically then consider applying BMPs to control factors further down the chain. It is often the case that the application of a single BMP is not sufficient to reduce diffuse pollution to acceptable levels. In this case it is usually better to apply additional BMPs to control a different step in the transport chain listed above, rather than to add another BMP that controls the same factor as the first BMP applied. As a simple rule of thumb, the best returns will be achieved if sources nearest to a river are tackled before those further away but this must be tempered by the relative size of different pollutant sources.
Effect on Particle Transport and Dissolved Runoff	Some BMPs only reduce the run-off of particles, other BMPs preferentially reduce dissolved components, others can reduce both components. The mode(s) of action of the BMP are always noted, i.e. does it reduce only the suspended solids content and any chemical attached to it, does it reduce mainly the dissolved pollutant or both? This is particularly relevant when considering the control of diffuse pollution of a pollutant for which the literature contains no previously tested BMPs. Knowledge of the mode of action will indicate BMPs which are likely to be successful for the new pollutant. Further information is available in the Siting and Suitability guidance
Efficiency	Efficiency values are reported in the literature in a number of ways. In order to simplify the collation of these data a common format has been used. For each of three forms of a pollutant (dissolved, particulate or total) a maximum efficiency and/ or a minimum value may be reported. A different value in the two columns indicates that a range of efficiencies was specified in the reference. Alternatively, a single value in one or other of the columns indicates that an upper or lower limit only was reported. The same value in both columns indicates that only one value for the efficiency was reported. Data are recorded as percentage reductions, unless otherwise stated in the comments. “N/A” is recorded in boxes for which the BMP is not applicable. An empty box infers that no relevant data has been located in the literature. The largest source of data is the US, where results are often reported in terms of effects on the Universal Soil Loss Equation (USLE) parameters. Since the dominant erosion processes in the UK are different, the USLE is not generally applicable in the UK (Naden, pers comm), although there are a few areas where it can be applied successfully (A Frost, pers comm) as a result, the transfer of efficiencies can be problematical. One complication with data reported in the literature is that many of the data were collected at the field scale and no account is taken of the likelihood that pollutant lost from a field will actually reach a river, ie the “connectivity” of the system. Only limited data exist on the efficiency of BMPs at the catchment scale (Ref Nos 48, 22). Numerical references can be followed up in the reference section under further information.
Cost	Data on costs are recorded as cash values at the date of the calculation (recorded), where they have been reported in the literature. Four components need to be considered: set up costs; the lifetime of any equipment bought or engineering carried out (or alternatively the frequency of repeating an operation); the running costs and the opportunity costs (for example, the value of the lost crop which could have been grown on a buffer strip). Any relevant grants which can be used to offset costs, should also be included. The SAC Farm Management Handbook is a good general reference of current costs.
Savings	Where definite savings have been quantified; they are included in this section. If no data are recorded it does not imply that no savings accrue from a BMP; simply that no data have been found. Although there are claims in the literature that many BMPs make significant savings for farmers the estimation of these are often optimistic or are very dependent on the circumstances on an individual farm. As a result the savings/cost benefit figures should be taken as a potential, which may, or may not be achievable on a particular farm. The true benefits of each BMP should be recalculated for each farm situation.
Cost Effectiveness Calculations	In situations where the savings claimed for a BMP are greater than the costs, a farmer cost-benefit analysis will be given (ie a cost benefit as seen by the farmer, which does not include the value of benefits or costs which accrue to the general public, for example by an improvement in water quality). However the comments made above apply. Where no clear cost-benefit to the farmer can be identified a cost effectiveness figure may be given, ie the cost per unit of pollutant removed. This allows different BMPs to be compared. The cost effectiveness of a BMP can vary a great deal, depending on the accounting assumptions made in the calculation. It is important that any cost effectiveness calculations are recorded in some detail so that the assumptions (eg write-off times for equipment, interest rates for borrowed capital, etc) can be reassessed at a later date.
Limitations	Some situations where the BMP will either not work or have reduced efficiency have been identified from general discussions. They should be taken into consideration when assessing the potential usefulness of a BMP in a particular catchment.
Maintenance	In some BMPs, approximate maintenance costs have been included.
Further Information	Includes a range of sources of additional information. Numerical references referred to in the efficiencies section are contained in References.

Pollution "Hot Spots"

On most farms, the majority of pollutants transferred arise from a relatively small proportion of the total farm area; these areas are frequently known as “hot spots”. There are BMPs appropriate to all the above in the handbook and targeting any hot spots is important.

For example, soil erosion can transfer small soil particles and adsorbed nutrients and pesticides from land to water. On most farms where soil erosion occurs, it is present only in limited areas, typically moderate or steep slopes with susceptible soils which are in arable cropping. Of the areas likely to erode, some are usually remote from surface water and eroded soil is simply deposited within the farm without causing any pollution. It is only where the eroded soil reaches a water course that pollution occurs. In such a case, erosion control measures may be targeted at the critical areas and low rates of soil erosion elsewhere on the farm can be tolerated.

Hot spots may include areas where stock may shelter from adverse weather, poorly constructed or maintained watering bays and troughs, routes which dairy cattle regularly walk especially river crossings, areas where sprayers are filled etc.

Septic Tanks

Septic tanks should only be used for domestic drainage. During Diffuse Pollution Audits on established farms, tests should be run to check that farm drainage is not passing through the septic tank. If non-domestic waste is passing through the tank, remedial action must be taken urgently. More information on maintaining septic tanks is available here.

Compatibility with Current Codes of Practice

As stated earlier this handbook contains as complete a list as possible of BMPs with agricultural applications. As such some are not recommended for use and are included simply as a source of reference for users who may wish to check on the usefulness of proposed BMPs they come across in the literature. These BMPs may or may not comply with current Scottish codes of practice.

Conversely, all BMPs “recommended” in this handbook for consideration as part of a package of BMPs in a particular situation, are consistent with advice given in current Scottish codes of practice and assurance schemes:

The Code of Practice approach and the BMP approach are somewhat different however. Codes of Practice contain series of measures and practices which all farmers are encouraged to adopt. The BMPs go far wider containing many alternative methods of addressing any given problem. In no way is it envisaged that any one farmer will adopt all or even many of the BMPs set out here. Rather they should be regarded as a tool kit from which tools may be selected as required to address any diffuse pollution problem.

Within the codes of practice are certain recommendations not included in the BMP handbook. Examples include recommendation on soil conditions for manure spreading or recommendations on the need for good record keeping. These might be regarded as BMPs but are not included here because they probably fall within the scope of normal good farming practice rather than special measures to address identified diffuse pollution problems.