specs

STORMWATER POND MODIFICATION
SOUTH NAPA MARKETPLACE
JUNE 24, 2011
PAGE 7
• < 113 CFU/100mL of E. coli,
• < 180 CFU/100mL of fecal coliform,
• Median concentrations of < 216 CFU/100mL of total coliform.
It is not anticipated that treated stormwater effluent from the proposed project will exceed these
maximum limits. The TMDL for sedimentation specifies no new regulations for urban
stormwater discharges; instead best management practices are required to decrease sedimentation
at the source, as is the case through the diligent housekeeping practices at South Napa
Marketplace. Nutrient levels of stormwater runoff were found to be very low (refer to Table 3),
again a reflection of the operating requirements and proscriptions at this shopping center. The
BMPs proposed in this application for modification will adequately address any remaining
nutrient levels that may be present. Thus, nutrient levels in the stormwater effluent are not
expected to add to the impairment of the Napa River.
Expected Effluent Water Quality
Anticipated effluent concentrations were estimated in Table 5 in Appendix A for the proposed
system using the documented removal rates for the treatment system elements discussed below.
Since the most current monitoring data for the existing system includes fairly low stormwater
inflow concentrations which may not be representative of the site’s actual first flush
concentrations, average national stormwater data for commercial properties was used (obtained
from the National Stormwater Quality Database, Version 1.1, taken from the report "A
Compilation and Analysis of NPDES Stormwater Monitoring Information" compiled by the
University of Alabama). However, actual data from the existing monitoring report was used for
the initial total petroleum hydrocarbons (TPH) concentration, as this value was higher than the
national average. This may be accountable to the presence of relatively new asphalt.
Effluent concentrations were calculated for stormwater exiting the Kristar Separator (Primary
Stage Treatment) using a conservative total suspended solids (TSS) removal rate of 60% and a
TPH removal rate of 53%. Effluent concentrations exiting the StormTech sub-grade chamber and
gravel media filter system (Secondary Stage Treatment) were estimated using removal rates of
80% TSS, 90% TPH, 53% total zinc (TZn), and 49% total phosphorus (TP). Bioswale removal
rates included 95% removal of any residual TSS, TPH, and TP, and 55% removal of TZn.
Refer to Appendix A for a report summary of the documented performance for each treatment
stage by which the expected effluent water quality was estimated.
Monitoring and Maintenance
Monitoring and maintenance of the proposed stormwater system will be greatly simplified, in
comparison to that of the South Pond. Regular maintenance will include sediment and trash
removal from the, and trash removal and vegetation management of the bioswale. Removal of
sediment from the Kristar Separator unit and the Isolator Row will also remove associated
contaminants.