Table 2.

Estimate of THM4 mean concentrations among U.S. community water systems in year 2011 post-Stage 1 D/DBPR and any year post-Stage 2 D/DBPR (see Excel Table S2 for underlying data).

Source water type	System size (population served)	Post-Stage 1 THM4 mean concentrationsa ($\mathrm{\mu }\mathrm{g}/\mathrm{L}$)			Percentage reduction from Stage 1 to Stage 2b,c	Post-Stage 2 THM4 mean concentrationsd ($\mathrm{\mu }\mathrm{g}/\mathrm{L}$)
		$\le 40\mathrm{\mu }\mathrm{g}/\mathrm{L}$	$>40\mathrm{\mu }\mathrm{g}/\mathrm{L}–80\mathrm{\mu }\mathrm{g}/\mathrm{L}$	Total	All systems	$\le 40\mathrm{\mu }\mathrm{g}/\mathrm{L}$	$>40\mathrm{\mu }\mathrm{g}/\mathrm{L}–80\mathrm{\mu }\mathrm{g}/\mathrm{L}$	Total
Surface water systems	$<\mathrm{10,000}$	22.5	54.7	39.0	9.2%	22.5	48.0	36.2
	10,000–100,000	23.6	52.4	34.2	9.2%	23.6	43.7	31.1
	$>\mathrm{100,000}$	23.4	49.4	33.4	7.2%	23.4	41.4	30.4
	Subtotal	23.4	51.0	34.2	—	23.4	42.9	31.1
Groundwater systems	$<\mathrm{10,000}$	6.3	56.4	10.9	1.4%	6.3	48.7	10.7
	10,000–100,000	11.0	53.8	17.1	1.3%	11.0	51.5	16.8
	$>\mathrm{100,000}$	16.4	50.5	19.6	1.7%	16.4	47.6	19.4
	Subtotal	10.2	54.2	15.2	—	10.2	50.0	14.9
All systems	Total	18.5	51.4	28.7	7.8%	18.5	43.7	26.4

Note: —, not applicable; D/DBPR, U.S. Environmental Protection Agency Disinfectants and Disinfection Byproducts Rule; THM4, trihalomethane-4.

^aStage 1 Disinfectants and Disinfection Byproducts Rule.

^bFrom Stage 2 economic analysis, Exhibit 5.23 with groundwater system reductions adjusted for nondisinfecting systems.

^cUsing the fifth and 95th percentile estimates of percent reduction from Stage 1 to Stage 2, the overall range of percent reduction was estimated to be 4.4%–11.2%, resulting in an overall range for the post-Stage 2 THM4 concentration of $25.5–27.4\mathrm{\mu }\mathrm{g}/\mathrm{L}$. Calculation of the post-Stage 2 concentrations is based on two assumptions: a) All Stage 2 reductions will occur in the $40–80\mathrm{\mu }\mathrm{g}/\mathrm{L}$ systems, and therefore the post-Stage 2 concentrations for the $<40\mathrm{\mu }\mathrm{g}/\mathrm{L}$ systems are the same as the Stage 1 concentrations; and b) For each source and size strata, the post-Stage 2 concentration for all systems is a population-weighted average of the concentrations in the two ranges of interest.

^dThe $>40\mathrm{\mu }\mathrm{g}/\mathrm{L}$ to $80\mathrm{\mu }\mathrm{g}/\mathrm{L}$ concentrations can be expressed as ${\text{Mean}}_{40–80}=\left(\text{Overall\hspace{0.17em}Mean}\times \text{Total\hspace{0.17em}Population}-{\text{\hspace{0.17em}Mean}}_{<40}\times {\text{Population}}_{<40}\right)/{\text{Population}}_{40–80}$. Using the data for the surface water systems serving $>\mathrm{100,000}$ people as an example to calculate the THM4 post-Stage 2 concentration for the $40–80\mathrm{\mu }\mathrm{g}/\mathrm{L}$ range results in: $41.4=\left(30.4\times 109.2\hspace{0.17em}-\hspace{0.17em}23.4\times 66.9\right)/42.3$.