STREAMFLOW GENERATION ON A SMALL AGRICULTURAL CATCHMENT DURING AUTUMNRECHARGE .1. NONSTORMFLOW PERIODS

Some sources and controls on nonstrom streamflow from a 19.8 ha agricu ltural hill land catchment located in east-central Pennsylvania were h ypothesized and identified based on O-18, Si, NO3, Cl and SO4 concentr ation patterns. Streamflow, springflow, soil water, seepage and shallo w groundwater were sampled routinely from 16 October to 20 November 19 89, which included three storms (17-20 October, 9 November and 14 Nove mber). The first storm caused this catchment to shift from a low to hi gh flow regime typical of the late autumn period. The chemical pattern s in streamflow followed a three-part sequence - stormflow, early post -storm drain-down period, and subsequent post-storm base flow period. The stormflow period was characterized by dilution of NO3, Cl, SO4, Si and O-18, whereas the highest NO3, Cl and SO4 concentrations and flow rates occurred during the early post-storm period, suggesting that ad ditional subsurface sources were temporarily activated by the storms. The chemical patterns and concentrations during base flow periods tend ed to be similar irrespective of preceding rainfall, which suggests th at subsurface storage is a key control on streamflow chemistry. Nonsto rm streamflow included discharge from springs, and near-stream seepage , and groundwater sampling sites located upstream. The spring and stre am chemistry were most similar, but differed from the shallow groundwa ter and seepage, which were nearly twice as concentrated in NO3. Thus, these sampling sites did not represent all nonstorm streamflow source s. The chemical indices most useful for characterizing the sources and behavior of nonstorm streamflow were NO3 (agricultural land), SO4, Cl and NO3 (storm-activated subsurface source areas). SO4 (spring contri butions), Cl and O-18 (dilution or mixing), Si (geochemically controll ed and stable - latter nonstorm period; dilution controlled - storm pe riod; kinetically controlled - early post-storm). The ratios of these chemical parameters were examined as indices; Si/Cl and SO4/NO3 were t he most interesting and provided additional tools and insights for cha racterizing hydrologic source areas.