posted on 2024-03-07, 14:35authored byAnthony J. Tesoriero, Susan A. Wherry, Danielle I. Dupuy, Tyler D. Johnson
Redox
conditions in groundwater may markedly affect the fate and
transport of nutrients, volatile organic compounds, and trace metals,
with significant implications for human health. While many local assessments
of redox conditions have been made, the spatial variability of redox
reaction rates makes the determination of redox conditions at regional
or national scales problematic. In this study, redox conditions in
groundwater were predicted for the contiguous United States using
random forest classification by relating measured water quality data
from over 30,000 wells to natural and anthropogenic factors. The model
correctly predicted the oxic/suboxic classification for 78 and 79%
of the samples in the out-of-bag and hold-out data sets, respectively.
Variables describing geology, hydrology, soil properties, and hydrologic
position were among the most important factors affecting the likelihood
of oxic conditions in groundwater. Important model variables tended
to relate to aquifer recharge, groundwater travel time, or prevalence
of electron donors, which are key drivers of redox conditions in groundwater.
Partial dependence plots suggested that the likelihood of oxic conditions
in groundwater decreased sharply as streams were approached and gradually
as the depth below the water table increased. The probability of oxic
groundwater increased as base flow index values increased, likely
due to the prevalence of well-drained soils and geologic materials
in high base flow index areas. The likelihood of oxic conditions increased
as topographic wetness index (TWI) values decreased. High topographic
wetness index values occur in areas with a propensity for standing
water and overland flow, conditions that limit the delivery of dissolved
oxygen to groundwater by recharge; higher TWI values also tend to
occur in discharge areas, which may contain groundwater with long
travel times. A second model was developed to predict the probability
of elevated manganese (Mn) concentrations in groundwater (i.e., ≥50
μg/L). The Mn model relied on many of the same variables as
the oxic/suboxic model and may be used to identify areas where Mn-reducing
conditions occur and where there is an increased risk to domestic
water supplies due to high Mn concentrations. Model predictions of
redox conditions in groundwater produced in this study may help identify
regions of the country with elevated groundwater vulnerability and
stream vulnerability to groundwater-derived contaminants.