*3.4. Delta Island Drainage Investigation*

Data collected under IDHAMP showed high THM formation potential in Delta island drainage. Delta island drainage refers to return flows collected from lands in the Delta and pumped into surrounding channels. Pumping of drainage is needed because organic-rich soils in the Delta lowlands are subsided relative to water levels in surrounding channels. Motivated by this finding, the Delta Island Drainage Investigation was initiated in 1987 to assess the impacts of Delta island drainage on the quality of drinking water supplies taken from the Delta [39]. Goals of the investigation were to (i) evaluate the quality and quantity of island drainage, (ii) identify processes that affect quality and quantity of island drainage, (iii) determine potential impacts of island drainage on water quality in Delta channels and at drinking water supply intakes, and (iv) explore potential mitigation strategies.

The investigation concluded that Delta island drainage had a higher potential to form THMs than water from Delta channels. While THM formation potential was found to vary from island to island, in general, drainage was found to have four times greater THM formation potential than Delta channel samples. Based on mass balance calculations, island drainage was estimated to contribute 40–50% of the THM formation potential in Delta waters during periods of irrigation and winter leaching [39]. Amy et al. [13], in conjunction with CDWR, corroborated the investigation's conclusion that Delta island drainage has a higher THM formation potential relative to the surrounding Delta channels. Through investigation of the chemical composition of water samples, the authors found distinct differences between Delta island drainage and channel waters, with drainage samples exhibiting higher average molecular weights. Amy et al. [13] also conducted a mass balance analysis and concluded that, on average, island drainage could contribute as much as 20% of the THM formation potential found in the SWP.
