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SECTION 4 water Quality Eualuatiou clarification /interpretation of rules with respect to criteria for separation distance to groundwater, it is unlikely that any of the drywells in the basin will meet criteria for rule authorization based on criteria g) and h) from above. An evaluation of the City and County drywells with respect to high groundwater and the presence of a filtration medium between the drywell bottom and the high groundwater was conducted for this basin plan and is provided in Appendix E. Seasonal high groundwater levels were found to be close to the surface in this basin (i.e., approximately 8 feet deep on average). Based on this evaluation, there are several drywells that are expected to be discharging directly to high groundwater, and there were not any drywells where a distance of 10 feet was expected between the bottom of the drywell and the high groundwater level. Therefore, it is likely that most or all of the County and City drywells will eventually require decommissioning. Both the County and City have applied for a WPCF permit with DEQ. A plan to decommission the drywells will be a part of the permit. The decommissioning plan will be based on the management alternatives evaluated and selected as part of this storm drainage master plan and as provided in Section 4.3. As decommissioning projects are being implemented, if further groundwater investigations reveal opportunities to rule authorize drywells, retaining and /or retrofitting selected drywells may be considered. As mentioned previously in Section 3, private drywells are under the authority of DEQ and any decommissioning associated with private drywells (if required) would be directed by DEQ. 4.2 Evaluation of Existing and Expected Future Water Quality Conditions This section describes water quality conditions in the basin in terms of both pollutant loads and stream stability issues. Pollutant Loads To supplement the general water quality information provided in Section 2.6, pollutant loads for total suspended solids (TSS) were calculated for this basin. Although TSS has not been shown to directly relate to all other pollutants, it was used as a general indicator of other pollutants for the purposes of making relative comparisons. The relative values of the TSS load were used to evaluate the impact of drywells on water quality, as drywells infiltrate runoff resulting in a net pollutant load reduction, and to highlight those land uses and drainage areas that appear to contribute the largest pollutant load to receiving waters. The values were also used to evaluate the relative contribution and increase in pollutant loads expected from future development. The methods used to estimate pollutant loads are described in Volume I, Section 3.2. The pollutant load estimates for the River Road Santa Clara basin are summarized in Figures 4 -1 through 4 -3 below. As mentioned in Section 2.6, these results are based on stormwater quality monitoring conducted in the City of Eugene. Although none of these data were collected from within the River Road Santa Clara Basin, they provide general information regarding stormwater quality in Eugene and were used in identifying a stormwater management strategy for this basin. The pollutant load estimates are based on the following assumptions: 1) new development would occur without the inclusion of water quality best management practices (consistent assumption used for the other basins, enabling comparison of pollutant estimates between basins): 2) during an average year, all flows from drywells (and, hence pollutant loads) would be infiltrated and would not discharge to surface waters; 3) all drywells were assumed to be located in residential areas; and 4) decommissioning of all drywells would result in those discharges 0:25695978 Eugene RR -SC Final Basin P1an\Master P1an\FINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 4 -4