Arsenic sampling data for each point-of-entry source were used as the outcome variable in our regression model, as described under "Regression Model" below.
Finally, we determined the average nitrate level for each point-of-entry source and averaged the resulting values across all sources to estimate an average systemwide nitrate level.
Because we did not have flow measurements for individual sources, we assumed that each point-of-entry source contributed independently, constantly and equally, to a CWS's distribution system, regardless of season.
Instead, we assumed that each point-of-entry source contributed independently and equally to a CWS's distribution system, and that each source contributed a constant amount to the system, regardless of season.
We included CWSs that were active in the San Joaquin Valley between 1999 and 2001, had at least one point-of-entry source with a nitrate sample reported for this period, and had any source (i.e., point of entry or not) with geographic coordinate data available to estimate CWS demographics.
We selected CWSs located in California's San Joaquin Valley that were actively operating between 2005 and 2007, had at least one source with a geographic coordinate that could be used to estimate customer demographics, and had at least one active point-of-entry source with an arsenic sample reported during this period.
Point-of-entry sources are those that directly enter the distribution system.
For CWSs with no confirmed arsenic treatment, we selected systems where either all point-of-entry sources were labeled as untreated, or all point-of-entry sources were labeled as having treatment.
However, sensitivity analyses, including and excluding CWSs with treated and untreated point-of-entry sources yielded consistent regression coefficients for home ownership.
Point-of-entry sources can be defined as sources of supply (e.g., well with no treatment or effluent from a well/surface water plant) that directly enter into the distribution system.
