Table 1.

Ecoregions with minimally affected background and >59 samples of specific conductivity (SC) measurements. Examples of 5th centile hazardous concentration (XCD₀₅) values are from extirpation concentration distribution (XCD) method, and the mean and 50% prediction limits (PL) using the background-to-criterion (B-C) method. Modeled mean base-flow SC is greater than 25th centile measured SC for all except Ecoregion 60. Figure 1 was used to select example values (gray).

		Source of Data Sets
		State data set			EPA-survey data set					Modeled
Level III ecoregion number and name		N	25th centile	XCD- Method XCD05	N	25th centile	B-C mean XCD05	B-C lower PL XCD05	B-C upper PL XCD05	Mean base-flow 25th centile
1	Coast Range	–	–	–	251	53	160	133	193	99
3	Willamette Valley	–	–	–	87	62	179	148	215	94
4	Cascadesa	–	–	–	537a	33	118	97	142	66
7	Central California Valleyb	–	–	–	82	99	243	202	291	411
11	Blue Mountains	–	–	–	135	85	219	182	263	161
15	Northern Rockies	614	29	142	29	22	89	73	108	100
16	Idaho Batholith	1040	42	185	29	36	125	103	151	81
17	Middle Rockiesb	510	136	264	89	78	208	173	249	229
19	Wasatch and Uinta Mountainsb,c	773	271	299	32	31	113	93	137	285
21	Southern Rockiesb	–	–	–	164	83	217	180	260	218
23	Arizona/New Mexico Mountainsb,c	374	191	249	38	105	253	211	303	385
35	South Central Plainsd	–	–	–	60	51	156	130	188	117
40	Central Irregular Plains	–	–	–	60	301	504	419	606	324
45	Piedmont	665	68	138	333	42	138	114	166	73
50	Northern Lakes and Forests	734	108	320	151	111	261	217	313	231
52	Driftless Areab,c	344	534	655	73	392	600	498	723	406
58	Northeastern Highlands	383	50	212	118	40	133	110	161	64
60	Northern Allegheny Plateauc	562	132	248	113	72	196	163	235	112
62	North Central Appalachians	–	–	–	131	33	118	97	142	60
65	Southeastern Plains (lower)d,e	457	38	131	241	26	100	82	121	88
65	Southeastern Plains (upper)c,d,e	501	99	243	241	26	100	82	121	88
66	Blue Ridge	322	22	69	245	16	74	60	90	59
67	Ridge and Valleyb	926	59	154	522	46	147	122	177	161
69	Central Appalachians	1661	94	305	281	46	146	121	176	94
70	Western Allegheny Plateau	2075	169	338	109	153	323	269	388	180
77	North Cascades	–	–	–	73	27	104	86	126	65
78	Klamath Mountains/California High North Coast Range	–	–	–	74	83	216	180	260	151

^aData set is a combined U.S. EPA survey and U.S. Geological Survey (USGS) data set ([data set] Cormier 2017a).

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

^cDifference of >50 μS/cm between state and U.S. EPA-survey background SC suggests need for further analysis.

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

^eElongated ecoregion suggests need for further analysis.