Analysis of groundwater migration from artificial recharge in a large urban aquifer: A simulation perspective

Abstract

Abstract. The increased use of reclaimed water for artificial groundwater recharge purposes has led to concerns about future groundwater quality, particularly as it relates to the introduction of new organic and inorganic contaminants into the subsurface. Here we review the development and initial application of a detailed numerical model of groundwater flow and migration in a region encompassing a large groundwater recharge operation in Orange County, California. The model is based upon a novel representation of geologic heterogeneity, which has long been known to influence local flow and transport behavior in the subsurface. The model and complementary series of isotopic analyses provide an improved scientific basis to understand flow paths, migration rates, and residence times of recharged groundwater, as well as to identify the source composition of water produced in wells near the recharge operation. From a management perspective these issues need to be confronted in order to respond to proposed regulatory constraints that would govern the operation of recharge facilities and nearby production wells. While model calibration is greatly aided by isotopic source and residence time analyses, the model also provides unique insights on the interpretation of isotopic data themselves. Isotopic estimates of groundwater age help discriminate between several equally acceptable simulations calibrated to head data only. However, the results also suggest that groundwater reaching a well spans a wide-ranging distribution of age, demonstrating the importance of geologic heterogeneity in affecting flow paths, mixing, and residence times in the vicinity of recharge basins and wells. BackgroundGroundwater is a significant source of drinking water in the United States and the rest of the world. Increased reliance on groundwater necessitates improved aquifer management with respect to understanding large recharge and discharge issues, planning rates of withdrawal, balancing demands of multiple users, and addressing water quality problems arising from industrial and agricultural contamination, artificial recharge, and saltwater intrusion.In California and elsewhere, artificial recharge practices are increasingly being used as a means •o augment groundwater availability and improve the overall reliability of water supplies. In this paper we are concerned with the fate and longterm water quality implications of artificial recharge in the large urban aquifer system underlying Orange County, California. We will explore the use of a detailed geologic modal of the aquifer system and numerical simulation as a basis to gain new scientific insight into these issues. We will also discuss how this approach has benefited from and contributed to similar efforts conducted from an isotopic analysis perspective. I and 3). Water recharged in the forebay can easily be drawn into the middle aquifers (Figure 2), as well as a shallow aquifer in the pressure zone and a much deeper "colored" water aquifer that is not used for production [OCWD, 1991]....

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“…Sedimentary deposits frequently consist of layers with varying grain size distributions that may cause the aquifer to behave locally as a multilayer system, where the actual flux distribution is not controlled as much by the hydraulic conductivity within the layers as by their continuity and inter-connectivity, particularly in the vertical dimension (Fogg, 1986;Martin and Frind, 1998). Characterizing heterogeneity in such systems at the recharge basin scale is required for proper representation of RTDs because heterogeneity causes uncertainty (Park et al, 2006) and promotes a broad range of residence time distributions (Tompson et al, 1999). But it is hard because the head differences are small and detailed hydraulic testing difficult to perform.…”