Abstract
Abstract. Non-point source (NPS) pollution has degraded groundwater quality of unconsolidated sedimentary basins over many decades. Properly conceptualizing NPS pollution from the well scale to the regional scale leads to complex and expensive numerical models: key controlling factors of NPS pollution – recharge rate, leakage of pollutants, and soil and aquifer hydraulic properties – are spatially and, for recharge and pollutant leakage, temporally variable. This leads to high uncertainty in predicting well pollution. On the other hand, concentration levels of some key NPS contaminants (salinity, nitrate) vary within a limited range (< 2 orders of magnitude), and significant mixing occurs across the aquifer profile along the most critical compliance surface: drinking water wells with their extended vertical screen length. Given these two unique NPS contamination conditions, we here investigate the degree to which NPS travel time to wells and the NPS source area associated with an individual well can be appropriately captured, for practical applications, when spatiotemporally variable recharge, contaminant leakage rates, or hydraulic conductivity are represented through a sub-regionally homogenized parametrization. We employ a Monte Carlo-based stochastic framework to assess the impact of model homogenization on key management metrics for NPS contamination. Results indicate that travel time distributions are relatively insensitive to the spatial variability of recharge and contaminant loading, while capture zone and contaminant time series exhibit some sensitivity to source variability. In contrast, homogenization of aquifer heterogeneity significantly affects the uncertainty assessment of travel times and capture zone delineation. Surprisingly, the statistics of relevant NPS well concentrations (fast and intermediate travel times) are fairly well reproduced by a series of equivalent homogeneous aquifers, highlighting the dominant role of NPS solute mixing along well screens.
Highlights
The use of agrochemicals to address an ever-growing food demand has led to the contamination of many sedimentary groundwater basins underlying intensively farmed regions (Nolan et al, 2002; Zektser and Everett, 2004; Rockstrom et al, 2009)
The homogenized predictions are least accurate during the transition period when concentrations in the vertically mixed sample obtained from a well are strongly controlled by travel time pdfs, which in turn are affected by the heterogeneity in the land use and aquifer dynamics
– Land use, soil, and aquifer heterogeneity lead to large variability in groundwater travel paths, travel times, source location, and well nitrate concentrations across a regional set of wells and, significant uncertainty about pollution dynamics at any one well
Summary
NPS pollution of groundwater is typically associated with dissolved solutes associated with groundwater recharge across the landscape Both recharge rates and contaminant concentrations in non-point sources are subject to large spatial and temporal variability. Conceptually simplified approaches have been successfully employed to predict general trends and expected (average) contaminant behavior across ensembles of pollutant receptors of interest (wells, stream reaches) (e.g., Conan et al, 2003) These assessments lack any measures to assess predictive uncertainties. We assess the degree to which detailed spatial representation of both the aquifer hydraulic conductivity and of contaminant source parameters – recharge rate of water and contaminant loading from the NPS to the groundwater table – can be homogenized in NPS models without reducing model accuracy. The later assumes structural ergodicity (Dagan, 1990), i.e., that the mean and variance of a single realization of the hydraulic conductivity field are close to the same statistics of the ensemble distribution (see the histograms in Supplement Fig. S1)
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