The Enterococcus Project: February 2, 2004
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Figure 3. 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. This is the same graph as Figure 4, except the enterococcus concentrations are plotted normally – using an arithmetic scale. It becomes clearer how much higher enterococcus levels are in lower Mission Creek than elsewhere. In both graphs the dashed line shows the permissible enterococcus level. As to which graph is better, it’s a trade off; a regular scale preserves the relative magnitude and impact of the data, while a logarithmic scale lets you see the actual values more clearly.