REDOX REACTION RATES IN SHALLOW AQUIFERS: IMPLICATIONS FOR NITRATE TRANSPORT IN GROUNDWATER AND STREAMS (Invited Presentation)

Groundwater age and water chemistry data along flow paths from recharge areas to streams were used to evaluate the trends and transformations of agricultural chemicals. Results from this analysis indicate that median nitrate recharge concentrations in these agricultural areas have increased markedly over the last 50 years from 4 mg N/L in samples collected prior to 1983 to 7.5 mg N/L in samples collected since 1983. The effect that nitrate accumulation in shallow aquifers will have on drinking water quality and stream ecosystems is dependent on the rate of redox reactions along flow paths and on the age distribution of nitrate discharging to supply wells and streams.

O₂ reduction and denitrification rates were determined by relating reactant or product concentrations to apparent groundwater age. O₂ reduction rates varied widely within and between sites, with zero-order rates ranging from < 3 µmol/L-yr to more than 140 µmol/L-yr. Moderate denitrification rates (10-100 µmol N/L-yr) were observed in most areas with O₂concentrations below 60 µmol/L, while higher rates (>100 µmol N/L-yr) occur when changes in lithology result in a sharp increase in the supply of electron donors. The availability of electron donors is the primary factor affecting oxygen reduction rates. When electron donors were limited, oxic conditions persisted for over 80 years of groundwater travel time and denitrification was not observed.

Redox reaction rates in groundwater influenced nutrient pathways to streams. For example, aquifers with low dissolved oxygen reduction rates resulted in groundwater that remained oxic from recharge to discharge. In these oxic watersheds, stream nitrate concentrations during base flow were high because oxic conditions inhibited denitrification in groundwater. Groundwater was the dominant source of nitrate in these streams, with inflows often more than 25 years old. These legacy sources of nutrients have important implications for assessing time lags between when changes in land use practices occur and when effects from these changes are observed.