ing hypothesis. We hypothesize that the DOC discharge-concentration relationship in the watershed with a peak concentration on the failing limb is partly controlled by the hydraulic properties of the riparian soil, the hydraulic characteristics of the hillslope, and by the storm-induced variation in stream stage. We report a combined modeling and field monitoring study of hydrologic interactions between stream water and groundwater on a watershed hillslope to address the questions related to DOC delivery from hillslope to streams. We used a physically based model to examine the role of lateral groundwater flow and groundwater and surface water interaction on the DOC export from the hillslope and how the vertical and horizontal movement of subsurface flow contributes DOC to the stream. Our model simulation results are consistent with field observations and with a previous study showing that the ma- jority of the DOC contribution to the stream is derived from the riparian zone (Mei et al., 2012). We used the model to explore the major factors controlling the DOC concentration-discharge relationships for a hillslope.
2 Methods
2.1 Field observations
during stormflow conditions (Mei et al., 2012).
An instrumented transect from the stream to the uplands was selected for the model and included wells on the flood plain (well 3, well 4), mid-hillslope (well 14), and the top of the hill (well 42) (Figure 15). The transect line is nearly perpendic- ular to the stream and parallel to the presumed flow direction of shallow phreatic groundwater. Wells 3 and 4 in the riparian zone are within 1 m of each other and parallel to the stream channel. We used hydraulic head measurements from well 3 and water chemistry measurements from well 4 to develop the model. Two soil types present along this transect include the Codorus series soils in the riparian zone and Edgemont series soils in the uplands. The bedrock below the soils is predominantly Setters Quartzite (Berg et al., 1980).
2.1.2 Field sampling and data collection
In July 2010, two instrument nests consisting of a pressure transducer and probes for soil temperature and soil moisture were established in the research transect, one in the riparian zone and the other in the uplands. Two soil moisture and temperature probes (Decagon device 5TM) were installed 10 cm below the ground surface in the instrument nests and pressure transducers were installed in wells 3 and 42. Nine piezometers had previously been installed in the stream bed at the bottom of the transect, 3 each at depths of 10cm, 30cm and 50cm (Battin et al., 2003). Stream water temperatures and well and streambed piezometer DOC concentrations have been measured for this transect over the past 2 decades. Soil texture analyses were conducted in 2003.
Pressure transducers were installed in a PVC pipe secured to the stream bed at the transect to collect stream stage data from July 2010. Data for peak flows proved reliable, but base flow data were not because of a temperature compensation issue with the transducer. The stream stage data from 2011 with a new pressure
Figure 15: Contour map of area around the research transect at White Clay Creek watershed, and the location of the wells. The red line represents our research transect with
length about 140 m long. The blue line represents the White Clay Creek stream channel.
The transect is located around the geographic coordination 395142.85N, 754655.26W.
transducer at base flow were compared to data from the outlet of the watershed (about 0.5 km downstream) for the same time period. A linear regression model was established using these two data sets and was used to estimate the base flow stage at the transect. Stream stage and groundwater level data were collected at 15 minute intervals, starting from July 8, 2010.
Stream water was collected during storms with an ISCO sampler (Teledyne Isco, Inc, Lincoln, Nebraska) in 2010. The sampler was programmed to take samples at a two hour time interval. Sampling was conducted during storms on July 12-13 (Event A), July 14 (Event B) and on July 19 (Event C). DOC samples from one storm (Event C) were taken from the sampler installed about 180m downstream of research transect and the rest of the DOC samples from storms were taken from the sampler at the transect. These two series of samples were considered to be consistent given the proximity of the two stream locations.
Samples on the hillslope were collected from well 4. The well was purged and sampled twice during the interval between storms (between Event A and Event B and 2 days after Event B before Event C).
2.1.3 Laboratory measurements
Water samples were filtered through pre-combusted (400C for 4 h) glass fiber filters (Whatman GF/F) using a peristaltic pump. Ten mL of filtrate were diluted 4X with deionized water. Additional volumes of filtrate were Tyndallized and kept in the refrigerator for biodegradable DOC (BDOC) analyses using plug-flow bioreactors with a bed volume of 600 mL (Kaplan and Newbold, 1995). BDOC was defined as the difference in DOC concentrations between the bioreactor influent and effluent waters. Refractory DOC (RDOC) was defined as the concentration of DOC in the bioreactor effluent. The Tyndallized stream water was diluted with biologically stable bioreactor effluent to a concentration of approximately 1.5 mg C/L prior to loading
onto the bioreactors (McLaughlin and Kaplan 2013, Biological lability of dissolved organic carbon in stream water and contributing terrestrial sources, submitted to Journal of Freshwater Science). A minimum of 3 bed volumes from each sample was discarded before samples of bioreactor effluent were collected for analysis. A total of two liters was needed for each bioreactor sample including the amount of water sent to waste, two inflow samples and two analyzed for DOC concentrations with a Sievers 900 analyzer equipped with an inorganic carbon removal module. The pump speed was set to 4mL/min. In total about 15 hours were needed for each run. A minimum of three bed volumes of baseflow stream water was run through the bioreactors between analyses of storm-water or well-water samples. The original RDOC and BDOC concentrations before dilution were calculated using the method given by Mei et al. (2012).
The BDOC analysis assumes that C is limiting (Kaplan and Newbold, 1995). To verify this, additional well samples were collected from well 4 on Aug 10, 2011, the samples were split into two groups, one amended with nutrients and the other una- mended. KH2PO4, Ca(NO3)2-4H2O and NH4Cl solutions were added to the nutrient amended treatment using the Redfield ratio (Redfield, 1958) as the composition. Both samples were diluted and run though the bioreactor following the standard procedure.
2.2 Model development