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Ground Water Stratification and Delivery of Nitrate to an Incised Stream under Varying Flow Conditions

^a USGS, 431 National Center, Reston, VA 20192
^b Dep. of Geology, Univ. of Maryland, College Park, MD 20742

View larger version (33K): [in this window] [in a new window]   Fig. 1. Map of the SERC-109 watershed, showing locations of temporary weirs (W1–W4), the permanent weir (W109), and the upper basin transect near the stream headcut (Peterjohn and Correll, 1984; O'Connell, 1998).

View larger version (28K): [in this window] [in a new window]   Fig. 2. Relation between stream flow and concentration of NO3– at stream sites in the SERC-109 watershed (see Fig. 1 for sample locations).

View larger version (35K): [in this window] [in a new window]   Fig. 3. Vertical section normal to the stream at the upper basin transect: (A) nested piezometer locations (HC1-HC9) with major features of the landscape and underlying geology; (B) logarithm of hydraulic conductivity at locations measured by slug tests.

View larger version (16K): [in this window] [in a new window]   Fig. 4. Design, results, and interpretation of a pump test for collection of CCl2F2 (CFC12) in a piezometer with slow recovery (HC1 at 12.5 m) (see Fig. 3 for location): (A) the piezometer was purged and then allowed to recover for 48 h by inflow of ground water through the screen at the bottom of the tube; (B) water was pumped out through a Cu tube from the bottom of the piezometer and samples were taken in the sequence 1 to 4 until the piezometer was empty; (C) analyses indicate that the first three samples were uniformly undersaturated with respect to atmospheric CFC12, whereas the last sample was somewhat nearer to saturation. Gas exchange between ground water and air in the piezometer during the recovery stage may have been limited to the top of the water column (Sample #4, the first to enter the piezometer during recovery).

View larger version (28K): [in this window] [in a new window]   Fig. 5. Hydraulic heads and apparent ground water ages in the upper basin transect: (A) hydraulic heads in April 1996; (B) CCl2F2 (CFC12) apparent ages in July 1996; (C) CFC12 apparent age gradients in July 1996, with additional data from "other" sites in the upper basin area (not all from the same time period). Heavy dashed lines in (C) indicate approximate limits of apparent age gradients and recharge rates (for porosity of 0.5).

View larger version (53K): [in this window] [in a new window]   Fig. 6. Distributions of selected solutes (A through F) in the SERC-109 upper basin transect, sampled 5 Apr. 1996. Additional NO3– data are given for selected samples from late April 1996 (italics). Concentrations are in µmol L–1. Zero values are <10 for NO3– and <50 for HCO3–.

View larger version (24K): [in this window] [in a new window]   Fig. 7. Dissolved gas data for piezometer nest HC9 beneath the cultivated field, indicating effects of denitrification: (A) concentrations of Ar and N2, with solid curves indicating aqueous gas concentrations in equilibrium with humid air for a range of temperatures (10–20°C) and addition of either 0 or 2 cm3 (STP) L–1 unfractionated air, and with shading indicating approximate concentrations assumed for recharge in the denitrified samples; (B) isotopic composition of dissolved N2 gas, showing the interpreted value for the denitrification component.

View larger version (38K): [in this window] [in a new window]   Fig. 8. Vertical variation of chemical and isotopic data at piezometer nest HC9, sampled 19–26 Apr. 1996 (Table 3). Data for excess N2 were derived from measurements of total N2 (Fig. 7). Measured values for selected sources of N (open circles) and S (open diamonds) are shown for comparison. X° and X indicate samples related by the reaction given in Eq. [4].

View larger version (27K): [in this window] [in a new window]   Fig. 9. Variation of 15N with concentration of NO3– in ground water and stream water under varying flow conditions. All data are from the SERC-109 watershed (Fig. 1). Values of 15N in the stream were relatively constant over a wide range of concentrations at high base flow and low base flow, but deviated toward precipitation values briefly during a storm event.

View larger version (21K): [in this window] [in a new window]   Fig. 10. Partial records in the upper basin area for (A) stream flows, (B) water table elevations in a near-stream piezometer, (C) stream and macropore NO3– concentrations, and (D) stream sulfate concentrations. The light curve in (A) indicates daily stream flow values at W109, divided by 10 (T. Jordan, personal communication, 2002).

View larger version (20K): [in this window] [in a new window]   Fig. 11. Relation between water table elevations in a near-stream piezometer nest and (A) stream flows, (B) stream NO3– concentrations, and (C) stream values of 34S[SO42–] (data were collected at irregular intervals between January 1993 and May 1996).

View larger version (40K): [in this window] [in a new window]   Fig. 12. Schematic diagram of ground water flow regimes contributing to stream flow under different flow conditions in the incised upper basin area of the SERC-109 watershed. Correlated variations in flow and NO3– concentrations in the stream are related to vertical stratification of hydraulic conductivity and redox status of the aquifer sediments.