posted on 2021-01-28, 21:13authored byGiovanni Castaldo, Ate Visser, Graham E. Fogg, Thomas Harter
Nitrate is one of
the most abundant contaminants in groundwater
globally, in the United States, and in California (CA). We studied
well construction information, water chemistry, stable isotopes, and
noble gases to understand how groundwater travel time and recharge
source and mechanism control nitrate concentrations in domestic wells
in the San Joaquin Valley (SJV), CA, a large semiarid, irrigated agricultural
region. Using nonparametric statistics, we find a decreasing trend
in nitrates with groundwater travel time and well depth. Samples collected
from wells that are closer to rivers and that show indications of
river water recharge, either low recharge temperature or low δ18O signature, have lower concentrations of nitrates than samples
with isotopic signatures indicating mixed source or local precipitation
recharge. The curbing effect of river water recharge on nitrate concentrations
in domestic wells is similar for direct river recharge and water applied
as irrigation. This suggests that irrigation with river water also
has a diluting effect that reduces the concentration of nitrate found
in groundwater. This conclusion supports the idea that flood-managed
aquifer recharge may be considered for remediation of groundwater
nitrate when designing replenishment of aquifers.