Aggregated cumulative county arsenic in drinking water and associations with bladder, colorectal, and kidney cancers, accounting for population served

BACKGROUND

Many studies neglect to account for variation in population served by community water systems (CWSs) when aggregating CWS-level contaminant concentrations to county level.

OBJECTIVE

In an ecological epidemiologic analysis, we explored two methods-unweighted and weighted (proportion of CWS population served by county population)-to account for population served by CWS in association between arsenic and three cancers to determine the impact of population served on aggregated measures of exposure.

METHODS

CWS arsenic concentration data for 19 states were obtained from Centers for Disease Control and Prevention (CDC) National Environmental Public Health Tracking Network for 2000-10, aggregated to county level, and linked to county-level cancer data for 2011-5 from National Cancer Institute and CDC State Cancer Profiles. Negative binomial regression models estimated adjusted risk ratios (aRR) and 95% confidence intervals (CI) between county-level bladder, colorectal, and kidney cancers and quartiles of aggregated cumulative county-level arsenic concentration (ppb-years).

RESULTS

We observed positive associations between the highest quartile of exposure, compared to the lowest, of aggregated cumulative county-level arsenic concentration (ppb-year) for bladder [weighted aRR: 1.89(1.53, 2.35)], colorectal [1.64(1.33, 2.01)], and kidney [1.69(1.37, 2.09)] cancers. We observed stronger associations utilizing the weighted exposure assessment method. However, inferences from this study are limited due to the ecologic nature of the analyses and different analytic study designs are needed to assess the utility that the weighted by CWS population served metric has for exposure assessment.

SIGNIFICANCE

Weighting by CWS population served accounts for some potential exposure assignment error in epidemiologic analysis.

Occurrences and changes of disinfection by-products in small water supply systems

The small water supply systems (WSSs) often report high concentrations of disinfection by-products (DBPs) in drinking water. In this study, occurrences of trihalomethanes (THMs) and haloacetic acids (HAAs) in Newfoundland and Labrador (NL), Canada, were investigated from 441 WSSs for a period of 18 years (1999-2016). The WSSs were divided into groundwater (GWP) and surface water (SWP) systems, which were further classified into eight sub-groups (P1-P8) based on the population served (≤ 100; 101-250; 251-500; 501-1000; 1001-3000; 3001-5000; 5001-10,000; and 10,000+, respectively). The DBPs with probable and possible carcinogenic forms were estimated. Overall, 31.1% of WSSs were GWP, in which averages of THMs and HAAs were 32.2 and 27.7 μg/L, respectively, while the SWP had averages of THMs and HAAs of 97.6 and 129.2 μg/L, respectively. The very small WSSs (P1-P3) of GWP had averages of THMs and HAAs in the ranges of 29.1-43.5 and 15.8-64.3 μg/L, respectively. The P1-P3 of SWP had averages of THMs and HAAs in the ranges of 92.6-112.8 and 108.0-154.0 μg/L, respectively, which often exceeded the Canadian guideline limits. If the samples represented the populations homogenously, the total populations exposed to THMs or HAA₅ above the guideline values would be in the range of 132.08-181.38 in thousands (30.3-41.6% of total populations). The probable and possible carcinogenic forms of THMs in GWP and SWP were in the ranges of 4.8-48.8 and 4.4-7.0% of THMs, respectively. In HAAs, carcinogenic forms were in the ranges of 82.6-98.4 and 97.6-98.7%, respectively. The findings indicated that the SWP might need further attention to better protect human health.