Better understanding agriculture’s effect on shallow groundwater quality is needed on the southern Idaho, Twin Falls irrigation tract. In 1999 and 2002-2007 we resampled 10 of the 15 tunnel drains monitored in a late-1960s study to determine the influence of time on NO3-N, dissolved reactive P (DRP), and Cl concentrations, and flow rates of shallow groundwater outflows.Since the late-1960s, an 8-fold increase in the dairy herd has driven shifts toward increased feed cropping, which, along with improved hybrids and production, increased inorganic and manure fertilizer use. The late-1960s to early-2000s period saw a consistent 1.4-fold increase in mean tunnel-drain outflow NO3-N concentrations (from 3.06 to 5.06 mg/L), a 10% decrease in mean Cl (from 49.2 to 44.2 mg/L), and an overall 14% decrease in DRP (14 to 12 µg/L). However, 3 of the 10 tunnels exhibited increased DRP concentrations during the period, and the rate of DRP increase was positively related to increasing encroachment of confined animal feeding operations or residential development. Decreases in tunnel flow between sampling periods were linearly related to corresponding increases in the fraction of sprinkler irrigation employed on lands drained by the tunnels (P = 0.01). Further conversion to sprinkler irrigation is unlikely to reduce tunnel drain NO3-N concentrations since the latter were unrelated to changes in sprinkler coverage. The amount and timing of applied N, and availability for crop uptake or leaching should be more carefully managed in these soils to prevent continued increases in groundwater NO3-N concentrations
