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Factors controlling nitrate fluxes in groundwater in agricultural areas
Author(s) -
Liao Lixia,
Green Christopher T.,
Bekins Barbara A.,
Böhlke J. K.
Publication year - 2012
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2011wr011008
Subject(s) - denitrification , groundwater recharge , groundwater , biogeochemical cycle , nitrate , environmental science , water table , aquifer , hydrology (agriculture) , leaching (pedology) , surface water , soil science , environmental chemistry , nitrogen , environmental engineering , soil water , chemistry , geology , geotechnical engineering , organic chemistry
The impact of agricultural chemicals on groundwater quality depends on the interactions of biogeochemical and hydrologic factors. To identify key processes affecting distribution of agricultural nitrate in groundwater, a parsimonious transport model was applied at 14 sites across the U.S. Simulated vertical profiles of NO 3 − , N 2 from denitrification, O 2 , Cl − , and environmental tracers of groundwater age were matched to observations by adjusting the parameters for recharge rate, unsaturated zone travel time, fractions of N and Cl − inputs leached to groundwater, O 2 reduction rate, O 2 threshold for denitrification, and denitrification rate. Model results revealed important interactions among biogeochemical and physical factors. Chloride fluxes decreased between the land surface and water table possibly because of Cl − exports in harvested crops (averaging 22% of land‐surface Cl − inputs). Modeled zero‐order rates of O 2 reduction and denitrification were correlated. Denitrification rates at depth commonly exceeded overlying O 2 reduction rates, likely because shallow geologic sources of reactive electron donors had been depleted. Projections indicated continued downward migration of NO 3 − fronts at sites with denitrification rates <0.25 mg‐N L −1 yr −1 . The steady state depth of NO 3 − depended to a similar degree on application rate, leaching fraction, recharge, and NO 3 − and O 2 reaction rates. Steady state total mass in each aquifer depended primarily on the N application rate. In addition to managing application rates at land surface, efficient water use may reduce the depth and mass of N in groundwater because lower recharge was associated with lower N fraction leached. Management actions to reduce N leaching could be targeted over aquifers with high‐recharge and low‐denitrification rates.