P.J. Chilton

Screening for long-term trends in groundwater nitrate monitoring data

Alarge body of UK groundwater nitrate data has been analysed by linear regression to define past trends and estimate future concentrations. Robust regression was used. The datasets showed too many irregularities to justify more traditional time-series approaches such as ARIMA-type methods. Tests were included for lack of linearity, outliers, seasonality and a break in the trend (by piecewise linear regression). Of the series analysed, 21% showed a significant improvement in the overall fit when a break was included. Half of these indicated an increase in trend with time. Significant seasonality was found in about one-third of the series, with the largest nitrate concentrations usually found during winter months. Inclusion of nearby water-level data as an additional explanatory variable successfully accounted for much of this seasonality. Based on 309 datasets from 191 distinct sites, nitrate concentrations were found to be rising at an average of 0.34 mg NO3 l−1 a−1. In 2000, 34% of the sites analysed exceeded the 50 mg l−1 EU drinking water standard. If present trends continue, 41% could exceed the standard by 2015. We explored an alternative to the previously proposed Water Framework Directive aggregation approach for estimating trends in whole groundwater bodies (the ‘Grath’ approach: spatially average then find the trend). We first determined the trends for single boreholes and then spatially averaged these. This approach preserves information about the spatial distribution of trends within the water body and is less sensitive to ‘missing data’.

Review of agricultural pollution in the Caribbean with particular emphasis on small island developing states

Recent studies have attributed the degradation of coastal living resources in the Caribbean to the potential impacts of agricultural pollution. Physical features controlling the delivery, retention and dispersal of pollutants throughout the region are discussed. Information relating to four types of agricultural pollution is presented and assessed: soil erosion leading to siltation, nutrient enrichment, pesticide contamination and agro-industrial pollution. The results of this review have enabled gaps in knowledge to be identified. Areas prone to soil erosion and the reasons for their susceptibility are known. There is a paucity of baseline data on turbidity and on the concentration of nutrients and pesticides in the coastal zone. The increase in the use of agricultural fertilisers and pesticides over the last 20 years suggests a concomitant rise in their loads to coastal waters. Few studies have made direct links between agricultural pollution, reduction in coastal water quality or clarity, and the degradation of coastal living resources.

Aquifers as environments for microbiological activity

Abstract The ability of an aquifer to support microbiological activity depends on several factors: the availability of nutrients and energy sources for microbial use; the physical characteristics of the aquifer itself (such as the groundwater flow regime); and the environmental conditions experienced by the organisms (such as temperature and pH). These factors will be reviewed and their implications discussed in relation to groundwater quality and to the geological disposal of radioactive wastes.

Pesticide fate and behaviour in the UK Chalk aquifer, and implications for groundwater quality

Field and laboratory studies have been used to assess the main factors that determine the fate and behaviour of agricultural herbicides in the Chalk aquifer of southern England. Field studies using isoproturon, chlortoluron and atrazine showed that leaching of pesticides from normal agricultural usage produces concentrations in Chalk groundwater of 0.01–1 μg l−1 for most compounds, which are comparable with the current EU drinking water standard of 0.1 μg l−1. Where significantly higher concentrations are found in groundwater (up to three or four orders of magnitude higher), these are associated with localized ‘point’ usage or disposal, often combined with more rapid preferential transport pathways to the water table. Studies of the degradation of isoproturon, mecoprop and atrazine showed that these compounds are significantly more persistent in the Chalk than in soils, with half-lives measured in hundreds rather than tens of days. The observed order of magnitude variation in isoproturon half-lives spatially and with time makes the choice of values for predictive modelling problematic. The implications of the results for the quality at groundwater supply sources are discussed and compared with the situation for nitrate, and uncertainties in knowledge, and therefore the requirements for further work on both transport pathways and pesticide persistence, are reviewed.

Pesticide pollution of the Triassic Sandstone aquifer of South Yorkshire

Diffuse pollutants such as pesticides pose a significant threat to groundwater quality. Following the wet autumn and winter of 2000–2001 an upturn in trace pesticide concentrations was detected in blended water drawn from part of the Triassic Sandstone aquifer. A groundwater sampling and monitoring programme was undertaken to assess whether this increase would continue. Over a period of 18 months, 190 groundwater samples were collected from the upper part of the aquifer and analysed for 40 pesticides and for nitrate. A total of 25 pesticides were found of which 16 were detected more than once. Positive pesticide detections were in excess of the EU maximum admissible concentration (MAC) for individual substances of 0.1 μg l−1 on 33% of occasions. The most commonly detected pesticides generally have higher agricultural application rates and/or relatively greater solubilities. Very high concentrations of mecoprop (up to 7.1 μg l−1) were consistently found at two of the sampling sites. Analysis of mecoprop enantiomers suggested more than one source for this contaminant. High concentrations of atrazine (up to 1.4 μg l−1) were also detected at three sites and high concentrations of isoproturon (1.2 μg l−1) were found where very high mecoprop concentrations were also present. Nitrate concentrations exceed the EU MAC of 11.3 mg l−1 N at four sites. The spatial and temporal distribution of pesticides obtained from the monitoring network shows no clear trends for prediction of future concentrations. Nitrate-N concentrations and pesticide detections show no clear relationship, suggesting different source, transport or degradation pathways.

Pesticides in groundwater: some preliminary results from recent research in temperate and tropical environments

Abstract The paper summarizes the preliminary results of field investigations of pesticide behaviour in the UK, Barbados and Sri Lanka. Field observations of groundwater pesticide concentrations in these studies range up to 100 µgl−1, but are more commonly less than 5 µgl−1. The risk of pesticide concentrations in pumped water reaching 10 or 100 times guideline values from normal agricultural use at recommended application rates is probably small. Simple laboratory determinations of degradation rates support earlier studies and general considerations of subsurface environmental conditions in suggesting that pesticides are likely to be more persistent in groundwater than in soils. Simple modelling of the saturated zone movement of pesticides towards wells indicates that concentrations in pump discharge are highly sensitive to aquifer porosity, pesticide half-life and extent of cultivated area. The greatest risk to groundwater from normal usage of pesticides in agriculture will occur where persistent compounds are applied over aquifers which are shallow, permeable and thin, and overlain by permeable soils.

Assessing the effectiveness of Scotland's groundwater nitrate monitoring network

Many countries expend considerable resources collecting and reporting data from national groundwater quality monitoring networks to assess diffuse nitrate contamination, but the reliability of these data depends on the effectiveness of the network. Without a representative monitoring network of reliable monitoring points, there is a risk that groundwater management policies could be developed and implemented based on poor evidence. To help increase confidence in nitrate data, a robust, practical and repeatable method was developed to assess the effectiveness of groundwater nitrate monitoring networks, and applied to a network in Scotland. The method combines a rapid site assessment of monitoring points and local pollution pressures in order to judge the reliability of monitoring points, with a wider characterization of the network to identify which land uses and hydrogeological environments are being monitored. The analysis indicates that, with minor exceptions, Scotlands groundwater nitrate monitoring network broadly represents the diversity of conditions expected in Scotland.

Groundwater recharge and pollutant transport beneath wastewater irrigation: the case of León, Mexico

Abstract Agricultural irrigation with wastewater is widely established in Mexico. The city of León is the centre of the Mexican tanning industry, and produces wastewater with a significant component of industrial effluent. The city is one of the fastest growing in Mexico and is highly dependent on groundwater for public supply. Much of this is drawn from aquifers around the city, including those beneath the wastewater area. The sources of recharge to these aquifers and the types of pollutants are reviewed. Regionally, groundwater levels are falling as a result of excessive groundwater abstraction but, within the area of wastewater irrigation, groundwater levels are maintained close to the ground surface. Groundwater sampling from existing boreholes, surface geophysics, soil sampling and the drilling of cored investigation boreholes have been able to characterize a body of shallow, polluted groundwater beneath the wastewater area, and contrast this with the regional groundwater quality. The most immediate threat to public supply from pollutants originating in the wastewater is posed by salinity. The predicted consequences of various management options for reducing the impact of wastewater on the quality of deeper groundwater are briefly discussed.