Table 2.

Ecoregions with minimally affected background and 23–59 specific conductivity (SC) samples. Modeled mean base-flow SC is greater than 25th centile measured SC. Examples of 5th centile hazardous concentration (XCD₀₅) values calculated with the lower 50% prediction limit (PL) of the background-to-criterion (B-C); therefore, XCD₀₅ values are first approximations. Figure 1 was used to assign example XCD₀₅ values (gray cells).

		Sources of Data Sets
		EPA survey					Modeled
Level III ecoregion number and name		N	25th centile	B-C Mean XCD05	B-C lower PL XCD05	B-C upper PL XCD05	Mean base-flow 25th centile SC
6	Central California Foothills and Coastal Mountainsab	27	244	439	365	527	480
8	Southern California Mountainsb	45	262	460	383	552	511
9	Eastern Cascades Slopes and Foothills	45	50	155	129	187	148
10	Columbia Plateau	26	128	287	239	344	264
13	Central Basin and Rangeb	51	152	321	268	385	367
18	Wyoming Basinb	37	397	605	502	728	443
20	Colorado Plateausb	33	436	643	533	775	495
25	High Plainsb	40	359	566	470	681	452
26	Southwestern Tablelandsb	25	495	699	579	844	544
29	Cross Timbersb	23	288	489	407	588	384
36	Ouachita Mountainsa	50	22	90	74	110	102
37	Arkansas Valleya	47	32	115	95	140	163
38	Boston Mountains	26	23	93	76	113	187
42	Northwestern Glaciated Plainsb	46	342	548	455	659	421
44	Nebraska Sand Hills	34	161	333	278	400	263
63	Middle Atlantic Coastal Plain	59	93	233	194	280	96
68	Southwestern Appalachians	55	40	132	110	160	104
74	Mississippi Valley Loess Plainsa,b	26	69	191	159	230	110
75	Southern Coastal Plaina	50	52	159	132	191	109
80	Northern Basin and Rangeb	47	94	234	195	282	269
82	Acadian Plains and Hills	23	53	161	133	193	89
84	Atlantic Coastal Pine Barrensa	37	62	179	149	215	68

^a([Na⁺] + [K⁺]) > ([Ca²⁺] + [Mg²⁺]) and/or [Cl⁻] > ([SO₄²⁻] + [HCO₃⁻]) in mg/L in ≥50% of U.S. EPA-survey sample.

^bPredicted mean base-flow varies by more than 200μS/cm in 20% stream segments in ecoregion.