Long-Term Monitoring of Riparian Water Table Depth and Groundwater Chemistry

Long-Term Monitoring of Riparian Water Tables and Groundwater Chemistry Long-term monitoring of riparian water tables and groundwater chemistry began in 2000 along four first or second order steams in and around the Gwynns Falls watershed in Baltimore City and County, MD. One site (Oregon) is in the completely forested Pond Branch catchment that serves as a "reference" study area for the Baltimore LTER (BES). Two sites (Glyndon, Gwynbrook) were in suburban areas of the watershed; one just upstream from the Glyndon BES long-term stream monitoring site in the headwaters of the Gwynns Falls, and one along a tributary that enters the Gwynns Falls just above the Gwynnbrook BES long-term stream monitoring site farther downstream. The final, urban site (Cahill) was along a tributary to the Gwynns Falls in Leakin Park in the urban core of the watershed. Water table data and more detailed descriptions of soils, vegetation, stream channel properties and microbial processes at these sites can be found in Groffman et al. (2002) and Gift et al. (2010). Water table wells (four per plot) constructed of 3.8 cm perforated PVC pipe were installed to 1.0 m below the ground surface in pairs perpendicular to the stream, each approximately five meters from the thalweg. The paired well transects are between 200 and 800 meters apart along the stream. Wells were installed by hand auguring to the desired depth, inserting the well casing, backfilling with clean sand to the top of the slots, then backfilling to within 15 cm of the surface with native fill, and filling the remaining hole with bentonite pellets. The wells were capped. In October 2010, the Oregon Ridge site wells were replaced with larger diameter 5.0 cm perforated PVC pipe installed to a depth of between 1.5 and 2 meters. There are now four wells at each transect at Oregon Ridge for a total of eight wells at that site. Water table levels are measured at one to four week intervals using a flashlight and folding rule. When wells were dry the maximum measurable depth (~1000 cm) was recorded. The well is first drained of standing water. Recharge time of a minimum of 10 minutes is given before collecting samples. The new deeper, larger diameter, wells at Oregon Ridge do not drain with the pump as the recharge is too strong so we instead draw off three liters of water from these wells before sampling. Samples for chemical analysis are pumped into a stoppered flask. The stopper of the flask is fitted with an attachment for a 60 ml HDPE bottle such that sample fills the bottle and then overflows into the flask. The sample in the bottle is used for chemical analyses while the overflow in the flask is measured for total volume. After one well has been sampled, the tubing through which the sample is pumped is rinsed with deionized water before the next bottle is attached for sampling for chemical analysis at the next well. Samples are stored with freezer packs while in the field. At the BES laboratory samples are syringe filtered using 25mm Whatman GF/F filters. Samples are refrigerated at 4 deg C and sent regularly to Cary Institute of Ecosystem Studies in Millbrook, NY for colormetric analysis of nitrate, ammonium and phosphate. Plot locations Oregon upper: 39 deg 29 min 12.69 sec N, 76 deg 41 min 15.45 sec W Oregon lower: 39 deg 28 min 52.83 sec N, 76 deg 41 min 15.71 sec W Glyndon upper: 39 deg 28 min 18.14 sec N, 76 deg 49 min 25.06 sec W Glyndon lower: 39 deg 28 min 18.93 sec N, 76 deg 49 min 9.92 sec W Gwynnbrook upper: 39 deg 26 min 39.55 sec N, 76 deg 46 min 40.27 sec W Gwynnbrook lower: 39 deg 26 min 36.31 sec N, 76 deg 46 min 50.95 sec W Cahill upper: 39 deg 18 min 33.88 sec N, 76 deg 41 min 2.56 sec W Cahill lower: 39 deg 18 min 30.62 sec N, 76 deg 41 min 5.96 sec W Literature Cited Gift, D. M., P. M. Groffman, S. S. Kaushal, and P. M. Mayer. 2010. Denitrification potential, root biomass, and organic matter in degraded and restored urban riparian zones. Restoration Ecology 18:113-120. Groffman, P. M., N. J. Boulware, W. C. Zipperer, R. V. Pouyat, L. E. Band, and M. F. Colosimo. 2002. Soil nitrogen cycle processes in urban riparian zones. Environmental Science and Technology 36:4547-4552.