Using regional groundwater flow models for prediction of regional wellwater quality distributions

This paper evaluates the use of standard regional groundwater flow models in predicting regional patterns of water chemistry based on wellwater samples. The regional groundwater flow in a moderately sized sandstone aquifer has been represented using a time-variant, three layer numerical model typical of those developed for water resource management. Reverse particle tracking was used to determine the implied age distribution of the water in the aquifer and the age structure and recharge location of the wellwaters at the time of sampling. At the regional scale, the model age predictions are broadly consistent with a pre-existing interpretation of wellwater chemistry, though the model suggests a complexity far greater than could be mapped using the 150 wells from which analyses were available. The main features of the chemical distribution pattern are controlled to a significant degree by the pre-abstraction flows within the aquifer, even though heavy pumping has occurred over a period of more than 100 years. The model suggests that some modern recharge is expected almost everywhere across the aquifer. despite its cover of low permeability Quaternary deposits. At individual wells, wide ranges in water age are often predicted, suggesting that water quality prediction for individual wells is likely to be uncertain, providing an explanation for locations where apparently anomalous wellwater chemistry occurs. We conclude that particle tracking using the implied flow field of a standard regional groundwater flow model has proved very worthwhile in the case studied. It has highlighted uncertainties in both flow and regional qualitative water quality interpretations, has confirmed or revised various explanations associated with water quality distributions, indicates that joint chemical and head calibration is advantageous, and suggests that broad predictions of future water quality changes might be attempted using the same approach, though predictions of the chemistry of individual wells is highly uncertain. The aquifer studied is typical of many sandstone aquifers, and we suggest that particle tracking investigations using existing regional-scale multi-layer, transient models might also be undertaken with advantage elsewhere. (C) 2010 Elsevier DV. All rights reserved.