Urban development in the upper Strawberry Creek watershed has produced a substantial amount of impervious surface area. Stormwater accumulates the debris and pollutants deposited by vehicles, pedestrians, commercial and institutional operations, atmospheric deposition, and a wide variety of other sources. These pollutants flow into the watershed storm drain system which is routed directly into Strawberry Creek.
Strawberry Creek is especially vulnerable to short-term urban pollutant "shock loading" because of the creek's limited assimilative capacity to attenuate these pollutants. Water quality degradation related to urban runoff is most pronounced following the first autumn rains when land surfaces are flushed of pollutants that have accumulated over the long dry season. The extent of wet weather water quality degradation depends on many variables such as extent and type of urban development, duration and intensity of the storm event, storm intervals, land surface porosity, and implementation of mitigative measures such as street sweeping.
Water quality sampling of Strawberry Creek was performed during the first significant rainfall (1.07 in) of the season on October 27, 1987. This was done to assess the impacts of urban runoff and to identify areas of non-point source pollution. Sampling methodologies and locations were identical to those used for the baseline monitoring program. Results of the storm sampling round are presented in Table 15. These will be compared to the baseline low flow water quality results and also to USGS and ABAG wet weather data for other Bay area creeks (Appendix B). Historical wet weather studies of Strawberry Creek (Table 9) will also be considered where applicable.
Dissolved oxygen concentrations were slightly lower than those found during the baseline study, although they were still high. Similar levels and slightly decreasing trends were observed in both forks. D.O. concentrations were similar to those reported in previous wet weather studies and also similar to average wet weather D.O. concentrations observed in other East Bay creeks.
Biochemical oxygen demand (BOD) increased slightly over baseline levels. BOD was still much lower than the average concentrations reported in other East Bay creeks during wet weather. In contrast, chemical oxygen demand (COD) increased significantly over low flow baseline levels and followed the same trends as BOD. COD was still lower than those levels reported by USGS for other East Bay creeks. The large increase in COD indicates that abundant non-biodegradable material such as plant matter was washed into the creek during wet weather.
In comparison to baseline levels, the dissolved solids content of Strawberry Creek decreased due to dilution of more naturally concentrated baseflow with runoff. However, suspended solids increased significantly in both forks, especially downstream. Downstream South Fork concentrations were higher than in the North Fork. Suspended solids levels approximately doubled in both forks through the course of the central campus. Downstream suspended solids concentrations were higher than average wet weather levels observed in other Bay area creeks.
Turbidity concentrations were also highly elevated over baseline levels and followed the same trends as suspended solids. Turbidity concentrations tripled in both forks along the course of the central campus. Similar high levels were observed in both forks. The turbidity concentrations in Strawberry Creek were higher than reported in previous studies, and also higher than average wet weather levels reported by ABAG for other Bay area creeks. Oil and grease concentrations increased slightly over baseline levels. Slightly higher amounts were found in the North Fork compared to the South Fork. A slightly increasing downstream trend was apparent in both forks. Oil and grease concentrations,were generally lower than the average wet weather levels reported for other Bay area creeks.
Total Kjeldahl nitrogen (TKN) increased significantly over low flow levels in Strawberry Creek. Similar concentrations and increasing downstream trends were observed in both forks. TKN was consistently higher than average wet weather levels found in other East Bay creeks. Ammonia concentrations moderately increased over baseline levels. Ammonia followed the same trends as TKN and was also consistently higher than average storm levels reported by the USGS for other East Bay creeks. Ammonia concentrations were lower than previously recorded wet weather observations, however. The large increase in TKN can be attributed mostly to organic nitrogen loading based on the low levels of ammonia present.
Nitrate concentrations actually decreased from the average baseline levels. Similar concentrations were found in all reaches of both forks. Nitrate was also similar to average wet weather levels reported in other East Bay creeks. The decrease in nitrate concentrations can be explained by the nature of the nitrogenous material and the flushing effect of the storm in flashy Strawberry Creek. Nitrate is the end product of the decomposition of organic nitrogenous matter. Although organic nitrogen loading during the storm was high, most of it was non-biodegradable material based on the results of BOD/COD analyses. In addition, the biodegradable matter did not have sufficient time to undergo nitrification because of the extremely short retention time of water in the creek under storm conditions. Existing nitrate-laden water was flushed downstream by the high peak storm flows in the creek.
Total phosphorus concentrations increased significantly over low flow baseline levels. Similar high concentrations and increasing downstream trends were observed in both forks. Phosphorus was generally higher than average wet weather levels reported for other East Bay creeks.
Total coliform bacteria concentrations in Strawberry Creek increased significantly over baseline levels. The highest concentrations were observed at the sampling sites located at the entrance to the central campus on both forks. North Fork levels were consistently higher than in the South Fork. Total coliform concentrations were similar to average wet weather levels observed in other East Bay creeks and generally higher than reported in previous studies.
Fecal coliform bacteria concentrations increased in the South Fork but decreased downstream in the North Fork compared to baseline levels. Fecal streptococcus concentrations increased in both forks over baseline levels. A large increase in both fecal coliform and fecal strep concentrations was observed in the South Fork at the Eucalyptus Grove and fecal strep counts were elevated in the North Fork at North Gate. Fecal coliform concentrations in Strawberry Creek were similar to levels reported for other East Bay creeks, whereas fecal strep concentrations were generally lower. The FC:FS ratios indicate that the fecal matter was of animal origin. The decrease in fecal coliform concentrations downstream in the North Fork was due to dilution of the normally high bacteria levels with surface runoff.
Trace metal concentrations in Strawberry Creek generally increased significantly over baseline levels. All metals increased with the exception of iron and manganese which decreased due to surface runoff dilution of their naturally-occurring levels. Increasing downstream trends were observed in both forks. Metal concentrations were generally much lower in the canyons than in the urbanized downstream reaches. In general, the North Fork had higher metals concentrations than the South Fork. The metals in Strawberry Creek were generally higher than average wet weather concentrations reported for other East Bay creeks. During the storm event, metals concentrations greatly exceeded the RWQCB standards for surface waters.
In summary, wet weather water quality of Strawberry Creek was impacted by urban runoff and deteriorated over baseline low flow water quality conditions. Significant increases in COD, suspended solids, turbidity, TKN, phosphorus, total and fecal bacteria as well as trace metals concentrations were observed compared to baseline levels. Dissolved solids and nitrate concentrations actually decreased due to dilution. Non-point sources of pollu on have a significant impact on the environmental quality of Strawberry Creek.