The catabolic activity of incumbent microorganisms in soil samples of eleven dissimilar soil series was investigated, with respect to the herbicide isoproturon. Soils were collected from a 30x37 km area of river catchment to the north-west of London, England. Catabolic activity in each soil type during a 500 h assay was determined by 14C-radiorespirometry. Results showed four soils that exhibited high levels of catabolic activity (33-44% mineralisation) while the remaining seven soils showed lower levels of catabolic activity (12-16% mineralisation). There was evidence to suggest that soils exhibiting high catabolic activity had low (<22%) clay content and tended towards lower organic carbon content (<2.7%), but that these higher levels of catabolic activity were also related to pre-exposure to isoproturon. The 14C-radiorespirometric results were used to produce a GIS layer representing levels of catabolic activity for the dissimilar soils across the study area. This layer was combined with other GIS layers relating to pesticide attenuation, including soil organic carbon content, depth to groundwater and hydrogeology, to produce a map showing risk of groundwater contamination by isoproturon. The output from this approach was compared with output from an attenuation-only approach and differences appraised. Inclusion of the catabolism layer resulted in a lowering of risk in the model in 15% of the study area. Although there appears to be limited benefit in including pesticide catabolic activity in this regional-scale groundwater risk model, this type of addition could be useful in a site-specific risk assessment.

Although environmental modelling is increasingly performed within a GIS framework, analysis of the associated error is far from routine, and rarely presented with the results. An important benefit of performing error analysis is its value in determining which elements of a vulnerability assessment framework need improving. With this in mind, it was decided to examine the extent to which error might propagate through a model of groundwater vulnerability to pesticide contamination. A pesticide leaching model was developed and incorporated into an assessment of groundwater contamination risk from normal agricultural use of the herbicide isoproturon, in a 30 km x 37 km area of river catchment to the north-west of London, England. The model, which comprised two main components accounting for (i) degradation and (ii) attenuation of the pesticide, was based on conventional contaminant transport calculations, combined with existing soil, rainfall, hydrogeological and depth to water table data. The results of an error analysis on the model were used to assign confidence limits to the resulting risk maps. In this instance, correlation of model variables led to a reduction of error in the final output. However, the results of the analysis showed how inclusion of low quality input data can lead to a large increase in output uncertainty. It is suggested that error propagation analysis should be routinely included in groundwater vulnerability assessment.

The 1991 EU Nitrate Directive was designed to reduce water pollution from agriculturally derived nitrates. England and Wales implemented this Directive by controlling agricultural activities within their most vulnerable areas termed Nitrate Vulnerable Zones. These were designated by identifying drinking water catchments (surface and groundwater), at risk from nitrate pollution. However, this method contravened the Nitrate Directive because it only protected drinking water and not all waters. In this paper, a GIS was used to identify all areas of groundwater vulnerable to nitrate pollution. This was achieved by constructing a model containing data on four characteristics: the quality of the water leaving the root zone of a piece of land; soil information; presence of low permeability superficial (drift) material; and aquifer properties. These were combined in a GIS and the various combinations converted into a measure of vulnerability using expert knowledge. Several model variants were produced using different estimates of the quality of the water leaving the root zone and contrasting methods of weighting the input data. When the final models were assessed all produced similar spatial patterns and, when verified by comparison with trend data derived from monitored nitrate concentrations, all the models were statistically significant predictors of groundwater nitrate concentrations. The best predictive model contained a model of nitrate leaching but no land use information, implying that changes in land use will not affect designations based upon this model. The relationship between nitrate levels and borehole intake depths was investigated since there was concern that the observed contrasts in nitrate levels between vulnerability categories might be reflecting differences in borehole intake depths and not actual vulnerability. However, this was not found to be statistically important. Our preferred model provides the basis for developing a new set of groundwater Nitrate Vulnerable Zones that should help England and Wales to comply with the EU Nitrate Directive.

An approach for defining land zones to protect groundwater resources from the effects of agriculturally derived diffuse nitrate pollution is described. The approach operates in two distinct stages. The first stage collates and statistically examines groundwater nitrate data from all available monitoring points. At the same time a number of groundwater vulnerability scenarios are modelled to determine the theoretical risks to groundwater based on land use. climate, soil and (solid and Drift) geological characteristics. The maps derived from this latter exercise are compared with the monitoring data to determine which of the scenarios best predicts the vulnerability of groundwater to nitrate pollution. In the second stage, the spatial distribution of nitrate concentrations is estimated by geostatistical analysis of the monitoring data and is used in conjunction with the selected vulnerability map to define land use zones. Development of the monitoring network and an approach to address anomalies are also described.

The Environment Agency in England and Wales has produced a Groundwater Protection Policy (GPP) which provides a consistent framework for the management and protection of groundwater. It uses a risk-based approach to decision-making with respect to groundwater resource and source protection that is supported by two practical tools, Groundwater Vulnerability Maps and Groundwater Source Protection Zones. Tracer testing can play an important role in validating these and other protection tools and this is demonstrated by three contrasting tracer test case studies. Each tracer test examines different elements of the framework used for assessing groundwater vulnerability and considers the impacts of point and diffuse pollution as well as microbiological contaminants.

Protection zones for wells are defined through the use of pathline tracing in ground water flow models. Traditional approaches to ground water now modeling focus on obtaining a single best model, occasionally supplemented by the use of sensitivity analyses, In most situations this approach is inappropriate because the ground water flow model is so poorly determined that a variety of different boundary conditions and parameter values would give similar predictions of heads, However, the range of feasible models may well give radically different predictions of the variable for which the model has been built. For example, the area, orientation, or many other descriptive variables of a catchment may be much more sensitive to the parameters used in a ground water now model than are the values of head commonly used to calibrate the model,This paper outlines a procedure to determine the range of predictions of catchments which would arise from alternative calibrations of a model, The range of catchments is used to identify zones of certainty and uncertainty, leading to alternative definitions of the protection zone for differing purposes, An example is presented, based on Bestwood Pumping Station, Nottinghamshire, which is located in the Triassic Sandstone Aquifer of the northern East Midlands of the United Kingdom, A trial and error calibration of a ground water flow model is used to determine an acceptable level for automatic calibration in the subsequent step. Only 12 measurements of ground water head were available, and these were used to determine the fit of a variety of model structures. A search routine revealed a range of feasible models and superposition of their catchments delineated the zones of certainty and uncertainty, which had areas of 7.4 and 15.2 km(2), respectively.

Protection zones for boreholes are defined through the use of pathline tracing in groundwater flow models. Traditional approaches to groundwater flow modelling focus on obtaining a single best model, occasionally supplemented by the use of sensitivity analyses. In most situations this approach is inappropriate because the groundwater flow model is so poorly determined that a variety of different boundary conditions and parameter values would give similar predictions of head. However, the range of feasible models may well give radically different predictions of the variable for which the model has been built, namely the borehole catchment. This paper outlines a procedure to determine the range of predictions of catchments which would arise from alternative calibrations of a model. The range of catchments is used to identify zones of certainty and uncertainty, leading to alternative definitions of the protection zone for differing purposes. An example is presented, based on Bestwood Pumping Station, Nottinghamshire, UK.