|Pionke h b,|
|De walle d r,|
Submitted to: Hydrological Processes
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/22/1994
Publication Date: N/A
Citation: Interpretive Summary: In order to control chemical losses from land to groundwater or streamflow it is critical to know from which land most storm runoff and soil drainage originates. Without this information, there is no practical or efficient way of focusing on the critical flow and chemical contributing areas within a landscape or watershed. This work addresses chemical and isotopic tracer use for delineating those critical areas. Presently, researchers are developing combinations of methods to trace timing or position in the watershed. One particularly attractive combination has been to exploit the differences between different natural occurring components including stable isotopes. Specifically, our work points out that chloride, nitrate, silica and sulfate may also be useful tracers of mixing, dilution, agricultural land use, and springflow contribution to streams. The addition of these tracers to those hydrologic and isotopic tools we already have provide us with a systematic way of analyzing chemical losses and control at the watershed scale. These are the kind of tools extension, SCS and consultant engineers will use to better decide land use management and position on watersheds.
Technical Abstract: Some sources and controls on nonstorm streamflow from a 20 ha agricultural hill land watershed located in east-central Pennsylvania were hypothesized and identified based on 18-O, Si, NO3, Cl and SO4 concentration patterns. Streamflow, springflow, soil water, seepage and shallow groundwater were sampled routinely from 10/16-11/20/92, which included 3 storms (10/17-20, 11/9, 11/14). The storm data is presented in DeWalle and Pionke (this issue). The first storm caused the watershed to shift from a low to high flow regime typical of the late fall period. The chemical patterns in streamflow followed a 3 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 18-O, whereas the highest NO3, Cl and SO4 concentrations and flow rates occurred during the early post-storm period suggesting activation of additional subsurface sources by the storms. Streamflow approached chemical stability during base flow periods irrespective of storm, suggesting subsurface storage to be the dominating control overall. Nonstorm streamflow included discharge from springs, and near-stream seepage and well sampling sites located upstream. Although the spring and stream chemistry were basically similar, both the shallow groundwater and seepage were chemically different, being nearly twice as high in NO3. Thus, these sampling sites did not represent all nonstorm streamflow sources. 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 contributions), Cl and 18-O (dilution or mixing), Si (geochemically controlled and stable--