Regional monitoring of temporal changes in groundwater quality

Changes in agricultural practices are expected to affect groundwater quality by changing the loads of nutrients and salts in recharging groundwater, but regional monitoring networks installed to register the changes often fail to detect them and interpretation of trend analysis results is difficult. This study aims to improve the detection and understanding of groundwater quality changes with time, combining time series information, concentration-depth profiles, age dating and concentration-depth prognoses based on the historical inputs of solutes. For trend detection, a combination of trend analysis on time series at specific depths and time-averaged concentration-depth profiles was used. To reveal trends that have become obscured by chemical reactions, additional conditionally conservative indicators were introduced that are insensitive to those reactions under specific conditions. Detected trends were matched with prognoses of conservative and reactive transport to aid the understanding of trends. Data of the regional networks in 2 area-types with intensive livestock farming in the Dutch province of Noord-Brabant were used to illustrate the approach. The downward movement of the agricultural pollution front was demonstrated for the 2 area-types. However, many targeted contaminants have become retarded or delayed and quality changes were hard to detect for many reactive solutes, including nitrate. Pollution fronts of these targeted chemical components are still limited to the first 15 m of the subsoil. At deeper level, about 20-25 m, the effects of agricultural pollution and acidification were indicated by chemical indicators that have not been considered by others: oxidation capacity, the sum of cations and chloride. Increasing trends of the conditionally conservative indicators 'oxidation capacity' and 'sum of cations' were found at a depth of 18-25 m below surface. Increasing trends for potassium were found at shallower depth (7-13 m), which is explained by retardation of potassium due to cation-exchange with calcium and magnesium. The modelled cation-exchange explained the shape of the concentration-depth profile and the increasing trends at shallow depth in the aquifer. © 2004 Elsevier B.V. All rights reserved.