1. Findings Is the City of Oberlin a source or a sink for pollutants? Water quality in Plum Creek as a function of urban land cover Jonathan Cummings, Tami Reed, and Kate Weinberger Systems Ecology (ENVS316) ’04 Oberlin College, OH Findings, continued References IntroductionMethod Conclusions and Take-Home Points 1. Turbidity and nutrient concentrations rise with increasing water flow rate 2. Oberlin tends to dilute Plum Creek during storm events. However, it tends to add nutrients and turbidity during low flow after storms. This is particularly true for turbidity and PO 4. Although the graph shows that Oberlin adds Cl under both conditions, it adds more during low flow. Along the same lines; while Oberlin always dilutes NO 2, 3 ; it dilutes more during high flow. 3.It is possible that the reason Oberlin dilutes Plum Creek during high flow is because other land cover/use farther upstream is adding more to the creek than Oberlin does. Two possible land uses are A) a construction site immediately above the upstream site and B) agricultural land cover even farther upstream. A construction site that may be contributing to increased nutrient concentrations at the upstream site. (photo courtesy of John Petersen) Background Plum Creek receives most of the water draining from the city of Oberlin, and eventually drains into the Black River and then Lake Erie Oberlin’s impermeable surface and utilization of storm drains may cause increased run-off into Plum Creek. Storm events are important determinants of short-term nutrient delivery in streams because they wash debris, sediments and nutrients into the water. The Plum Creek’s pollutant level is of interest to the town of Oberlin, the Black River Watershed Project and Lake Erie due to potential eutrophication effects A 1992-1994 EPA study concluded that the Plum Creek is an impaired water due to agricultural inputs and siltation 1 Previous studies have not evaluated the role of storms on water quality in Plum Creek 2. Upstream Plum Creek Sampling Occurred Under Pictured Bridge (photo courtesy of John Petersen) Objectives/Hypotheses We will determine the role of storm events on water quality upstream (above image) and downstream (below image) of the city of Oberlin during and after a storm event. We hypothesize that downstream nutrient concentrations will be higher than upstream concentrations due to urban land use. We also hypothesize that the difference between concentrations will be more pronounced during high storm flow. Downstream Plum Creek Sampling Site Sampling Sites We collected water samples from two sites to determine Oberlin’s effect on Plum Creek. The first site is located on Pyle Road, upstream of all of Oberlin’s storm drains. The second site is located on Route 511, downstream of all of the drains. Sub-watersheds for each sampling site. The area in dark blue drains to both sites, while the area in light blue drains only to the downstream site. Notice that the light blue area predominantly drains the city of Oberlin. Procedure We set up two ISCO automatic samplers, one at each site, to collect water. We programmed them to collect water samples at intervals of two hours for two days, and then at intervals of four hours for four days. We began sampling on November 3 rd and ended on November 10 th. A storm event occurred on the second day of sampling (November 4 th ). No precipitation entered the creek during the remaining days of sampling. We also took depth measurements at the upstream site during the sampling period as an indicator of water flow rate Immediately upon collecting each batch of water samples, we analyzed them for turbidity. We then filtered and froze the samples for later analysis Later, we analyzed samples for a number of nutrient ions, including: NO 2 and NO 3 PO 4 Cl (not a nutrient) We did not perform any statistical analyses, because we only sampled during one storm event and thus did not have any repeat samples Results We used our depth readings to plot a hydrograph of the storm event: Depth readings from the upstream site plotted against a time line. Depth is a rough indicator of flow rate (i.e., higher depth means faster flow). We found that nutrient concentrations and turbidity measurements at both sites increased during or immediately following the storm event. During the four days after the storm, nutrient concentrations either dropped to pre-storm levels or remained constant. We assume that in the latter case, concentrations would eventually return to pre-storm levels. The following graph shows how Oberlin’s effect on Plum Creek varies depending on flow: Median differences between downstream and upstream concentrations (downstream minus upstream). The high flow bars represent the storm; the low flow bars represent post-storm concentrations. Values are given as percentages of the maximum concentration for each nutrient (i.e., Cl values are given as percentages of the maximum measured concentration of Cl). When bars are positive, Oberlin is adding nutrients/turbidity to Plum Creek. When bars are negative, Oberlin is diluting the creek. This does not mean that Oberlin takes nutrients out of the water; rather, run-off from Oberlin is less concentrated than water that is already in the creek. 1 United States Environmental Protection Agency. 1998. Total Maximum Daily Load: Listed Water Information. 2 Fessenden, R.B. and Timberlake, R.T. 2000. “Plum Creek Water Quality” Report for Oberlin College ENVS 316. Upstream site Downstream site City of Oberlin 50 60 70 80 90 100 11/311/511/711/911/11 Date Depth Upstream depth vs. time (downstream - upstream) as a % of maximum Oberlin Dilutes Median difference in concentration between sites concentration of each nutrient -20 -10 0 10 20 30 40 50 60 70 TurbidityClPO4NO2,3 nutrient median difference as a % of maximum concentration of each nutrient During High Flow During Low Flow Oberlin Adds Oberlin Dilutes