4.1.2. Data Collection & Analysis

The MWQI program has conducted discrete water quality monitoring in the Delta since its inception in 1990 [25]. The spatial and temporal extent of this monitoring has varied in response to program needs and system understanding [25]. Currently (as of 2022), the program conducts routine discrete monitoring at ten locations (see Figure 1) and collects data at other locations in support of special studies. Discrete sampling has typically been conducted at a monthly interval and currently measures organic carbon (total and dissolved), standard minerals (i.e., major anions and cations), bromide, nutrients, and chlorophyll. TOC and dissolved organic carbon (DOC) are quantitative indicators of NOM content. Throughout its history, the MWQI program has evaluated several additional water quality parameters as part of routine monitoring and special studies, including trihalomethane (THM) formation potential, ultraviolet absorbance at 254 nm (UV-254) (a surrogate for the humic portion of the NOM), metals, selenium, pesticides, and herbicides [25].

The discrete water quality monitoring network, the backbone of the MWQI program in its early years, expanded dramatically at program inception. While IDHAMP had maintained a network of 15 to 18 sampling locations [39], in 1990, the MWQI program was collecting water quality samples at about 40 Delta urban intake and channel locations and about 30 agricultural drainage locations [43]. Discrete sampling frequency was typically monthly. Foreshadowing the program's later adoption of real time monitoring, beginning in 1993 the program experimented with the use of autosampler technology to increase sampling frequency in a cost-effective manner. These devices were programmed to collect samples at variable frequencies (i.e., daily and sub-daily), which were subsequently analyzed in a laboratory. Autosamplers were later replaced by auto-analyzers, devices that could analyze selected water quality constituents in near real time (typically within minutes). Highlights of the program's data collection and analysis efforts are summarized below; published program annual reports provide greater detail [43,53–56]. During this period, a parallel effort was undertaken by others [57] to evaluate loadings of key drinking

water contaminants to Delta tributaries and to determine if there were source control measures that, if implemented, would improve drinking water quality at Delta intakes.

Measurement of cations and anions in Delta source waters continued under the MWQI program, thereby providing continuity with previous IDHAMP monitoring. The intended purpose of these data was to characterize the major water types in the Delta (e.g., freshwater inflows, seawater intrusion, and agricultural drainage) and their sources. By developing a chemical fingerprint or profile of specific water types, the program planned to assess the movement and degradation of water under specific hydrologic conditions in the Delta. Cation and anion data also assisted modelers and planners in the examination of alternatives to improve the management and distribution of Delta water supplies [43]. A comprehensive analysis of these data, along with related legacy data available on CDWR's Water Data Library website http://www.water.ca.gov/waterdatalibrary/ (accessed on 1 November 2020), was undertaken more recently [58,59].

During this period, several advancements were made in the measurement and understanding of THM precursors and THM formation potential and their relationship to actual THM formation in treated water. Measurements of source water THM precursors, such as bromide, TOC, and DOC became routine. The resulting data showed consistent relationships between TOC and DOC and confirmed that seawater intrusion was the primary source of bromide in Delta waters [15]. Starting in 1990, water samples were also measured for UV-254, another indicator of NOM content, which was found to correlate with DOC in most water samples. UV-254 measurements were conducted for several years, with the intent of providing a quick and inexpensive measurement useful in assessing THM precursor levels in the Delta. In 1992, a modified chemical testing procedure was developed and adopted to improve measurement of THM formation potential in high DOC water samples [53]. This modified procedure was needed because the original THM formation potential assay method was shown to underestimate precursor levels in high DOC samples common in agricultural drainage [60]. Analysis of the program's THM formation potential data revealed a consistent relative distribution of the four THM compounds as a function of bromine incorporation factor [61] and strong correlations were found between precursor concentrations, THM formation potential data, and THM formation in simulated distribution system samples [62,63]. Krasner et al. [21] studied water samples from the Delta and other locations to evaluate research approaches for characterizing NOM, including its source and nature in watersheds, its response to seasonal variations, and its potential to form DBPs.

The relationship between Delta agricultural drainage and Delta source water quality became clearer as the MWQI program maintained and expanded monitoring initiated under the Delta Island Drainage Investigation program. The volume and quality of drainage was found to correlate with seasonal farming activities and regional soils. High drainage volumes were associated with farming activities centered on two periods. In the late fall and early winter, fields are flooded to leach out salt accumulations from the soil, resulting in high drainage volume and high DOC concentrations in the drainage, especially from organic soil areas. The second peak drainage period was observed during summer irrigation. DOC concentrations were found to be lower during the irrigation peak relative to the leaching peak; this difference was thought to be caused by less soil-to-water contact time and a lower water table that resulted in lower soil moisture. High DOC and THM formation potential levels were found to be associated with the organic content of the drained soils. The highest concentrations were typically found in drains located on peat soil areas and the lowest from mineral soil areas [53]. Following statistical analysis of drainage data and soil classifications proposed in CDWR [53], characteristic monthly values of DOC, THM formation potential, and UV-254 were proposed for organic soils (i.e., high-range DOC soils), intermediate organic soils (i.e., mid-range DOC soils), and mineral soils (i.e., low-range DOC soils); these are documented in CDWR [64] and summarized in Figure A2 (see Appendix A).
