The Enterococcus Project: February 2, 2004
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Figure 4. This chart show all Mission Creek results before, during and after the storm, up until the morning of the third day. Military time is shown along the bottom axis: 18:00 hrs is 6 PM, 0:00 is midnight. Flow is shown in the background; unfortunately this is not Mission Creek flow (the US Geologic Survey gauge at Rocky Nook Park is malfunctioning) but flow in Maria Ygnacio Creek at University Ave (we will have Mission flow data from UCSB sometime next week). Even though it’s the wrong creek, the pattern of flow – the pattern is called a “hydrograph” – at Mission would look much like this. It shows when the creek began to rise, the peak of the storm, and when the rain stopped and the water level began to drift back down to where it started from. The worse contamination occurred at Montecito Street at the initial peak of the storm – this was mostly urban runoff. Note that when the final storm peak occurred, at about 20:00 or 10 PM, concentrations at Montecito actually went down. After streets, paved areas and roofs are flushed by earlier rainfall, later storm pulses feed much cleaner runoff into the stream. Notice that after midnight – after rain had stopped and creek levels began to decrease – enterococcus concentrations began to rise. This is especially noticeable in Mission Cyn. and Rattlesnake. This is probably caused by riparian area soilwater leaking back into the creek as the water level declines, bringing with it bacteria from the soil. Finally, a word of warning about this graph. The vertical axis on the left has a logarithmic scale: in effect, the logarithms of bacterial concentrations are plotted instead of actual concentrations. It’s a way of showing both small and large numbers on the same graph. This way of plotting information minimizes the difference between numbers and differences that look small are, actually, quite large.