WORK PLANS - Laserfiche WebLink

Home Browse Search

Work Plan and Time Schedule for Analyses of Background Groundwater Quality City of Stockton Regional Wastewater Control Facility Page 6 low. Conversely, this well yields background values for salinity that are too low. MW-15 is located relatively close to the SJR with a shallow screen and is influenced by river water irrigation. It has a lower average TDS than wells with deeper screens near the ponds (see Figure 4). A TDS limitation based on data from MW-15 would identify inappropriately low background salinity criteria for other wells. 7.1.3 How is the background value to be determined, (e.g., upgradient wells, historical data, statistical evaluation of data set)? West of the SJR no true upgradient sites exist. The oxidation ponds and engineered wetland sit on a groundwater mound contained within the groundwater interceptor drain system. The facilities operated for decades before monitoring began in 2003. To establish pre-discharge background outside the interceptor drain system (Question No. 2), a range of background constituent concentrations typical of similar settings outside the areas of influence of the pond discharge will be derived. To accomplish this, many one-time samples of groundwater will be collected. Samples will be distributed over a similar area as the ponds and also over potentially impacted downgradient areas. These data will be analyzed statistically to calculate the expected range of local background water quality downgradient of the ponds. If the background thus determined is of higher quality than areas downgradient of the facility we can conclude that RWCF past practices have resulted in a degradation of water quality. If the background thus determined is of equal or lower quality than groundwater currently downgradient of the facility, we can conclude that RWCF discharges have been beneficial to water quality, and it would be consistent with State Board Policy 68-16 to derive a present-day background(see Section 7.1) for future monitoring. As discussed in Section 5.0, groundwater salinity indicators from all wells are not from a single population. This fact argues for establishing different present-day background threshold values for different areas of the facility. Statistical analysis of the geochemical database revealed that within each well EC Field data are normally distributed, a feature that argues for intra-well comparisons in which a stable(non-trended)background history is compared to new data at each well. The Regional Water Board has expressed a preference for avoiding intra-well comparisons and so, where possible, it is desirable to define specific numerical concentrations that apply to groups of wells. Statistical analysis of variance (ANOVA) of the existing database will be used to identify groupings of monitoring points that can be linked by their variance characteristics. If for example wells such as MW-15, MW-16, and MW-18 have overlapping distributions,are similar in variance,and their combined datasets form a normally distributed population (which they are for TDS data), then we can treat data from these three wells as a single population for statistical purposes. If it is possible to expand the "present-day" background population in this manner, we can provide a stronger basis for developing Upper Confidence Limits (UCL), Upper Tolerance Limits (UTL) or other present-day background values supported by traditional parametric statistical treatments described in California regulations (Title 27 §20415(e)8)(A)). Otherwise graphical techniques for intra-well comparisons, as allowed by California regulations, will be applied (Title 27 §20415(e)8). East of the SJR, upgradient locations can be found along the SJR levee, such as MW-11 and MW-14. Statistical testing of data from these two wells shows that when grouped together, they are normally distributed for EC Field, but not TDS (Appendix 1). Insufficient data now exist at these wells to establish normally distributed populations for all constituents. Parametric statistical methods using data from wells MW-11 and MW-12 will be applied, where feasible. However, groundwater chemistry typically changes with overlying land use and subsurface flow distance, increasing in salinity downgradient due to dissolution of soil mineral constituents and with depth as recharge dilutes the upper groundwater zones. Because of these processes, it is unlikely that groundwater from various locations equidistant from the SJR, and also samples along a groundwater flow path perpendicular to the river, are now or were ever of the same statistical population. If true, setting background thresholds based on data from wells MW-11 and MW-14 along the river would not account for water quality changes that naturally occur with A i41 CONDO