Table 1.

Number of U.S. community water systems and populations served by strata in year 2011 post-Stage 1 D/DBPR (see Excel Table S1 for underlying data).

Source water type	System size (population served)	Number of systems with THM4 mean concentrationsa			Population served by systems with THM4 mean concentrations in ranges indicated (millions)
		$\le 40\mathrm{\mu }\mathrm{g}/\mathrm{L}$	$>40\mathrm{\mu }\mathrm{g}/\mathrm{L}–80\mathrm{\mu }\mathrm{g}/\mathrm{L}$	Total	$\le 40\mathrm{\mu }\mathrm{g}/\mathrm{L}$	$>40\mathrm{\mu }\mathrm{g}/\mathrm{L}–80\mathrm{\mu }\mathrm{g}/\mathrm{L}$	Total
Surface water systems	$<\mathrm{10,000}$	4,213	4,748	8,961	8.2	9.5	17.7
	10,000–100,000	1,404	938	2,342	40.3	23.9	64.2
	$>\mathrm{100,000}$	242	111	353	66.9	42.3	109.2
	Subtotal	5,859	5,797	11,656	115.4	75.7	191.1
Groundwater systems	$<\mathrm{10,000}$	35,009	2,821	37,830	26.8	3.1	29.9
	10,000–100,000	1,293	186	1,479	28.8	4.9	33.6
	$>\mathrm{100,000},000$	61	8	69	13.3	1.4	14.7
	Subtotal	36,364	3,014	39,378	68.9	9.3	78.2
All systems	Total	42,223	8,812	51,034	184.3	85.0	269.3

Note: D/DBPR, U.S. Environmental Protection Agency Disinfectants and Disinfection Byproducts Rule; MCL, maximum contaminant level; PWS, public water system; THM4, trihalomethane-4.

^a PWS with THM4 mean concentrations $>80\mathrm{\mu }\mathrm{g}/\mathrm{L}$ were assumed to reduce their concentrations to achieve compliance with the MCL over time and included in the $40–80\mathrm{\mu }\mathrm{g}/\mathrm{L}$ range bin with the average concentration for systems in that bin.