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. Author manuscript; available in PMC: 2020 May 26.
Published in final edited form as: Sci Total Environ. 2018 Feb 1;633:1647–1656. doi: 10.1016/j.scitotenv.2018.01.137

Table 3.

Ecoregions with least disturbed background and at least one data set with >20 specific conductivity (SC) samples. Examples of 5th centile of the extirpation concentration distribution (XCD05) where 25th centile (SC) is greater than the geophysical modeled mean base-flow suggests background represents least disturbed conditions. XCD05 calculated by XCD method and the mean and 50% prediction limits (PL) using the background-to-criterion (B-C) method. Figure 1 was used to select example XCD05 values (gray).

Sources of Data Sets
State-survey EPA-survey Modeled
Level III ecoregion number and
name		 N 25th
centile
μS/cm		 XCD-
method
XCD05
μS/cm		 N 25th centile
μS/cm		 B-C
mean
XCD05
μS/cm		 B-C
lower PL
XCD05
μS/cm		 B-C
upper PL
XCD05
μS/cm		 Mean
base-flow
modeled SC
(μS/cm)
27 Central Great Plainsd – – – 133 469 675 560 815 452
39 Ozark Highlands – – – 54 362 569 472 685 278
43 Northwestern Great Plainsa,b,d – – – 399b 542 742 614 897 489
47 Western Corn Belt Plainsc 473 587 934 178 480 685 568 827 349
51 North Central Hardwood Forestsc 583 325 494 35 149 317 264 381 301
54 Central Corn Belt Plainsc 469 626 813 14 465 (465–1100) 671 556 809 267
55 Eastern Corn Belt Plains – – – 25 612 804 665 973 247
59 Northeastern Coastal Zonea,c 277 350 706 41 105 252 210 303 72
61 Erie Drift Plain – – – 33 230 422 352 507 166
64 Northern Piedmont 710 150 227 92 114 266 222 319 101
71 Interior Plateau 336 296 479 56 299 502 417 603 236
72 Interior River Valleys and Hillsc 463 462 1127 51 384 592 491 713 264
73 Mississippi Alluvial Plaind – – – 27 132 292 244 351 121
83 Eastern Great Lakes Lowlandsc,d 591 272 525 17 104 (104–1890) 250 208 300 181
a

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

b

Data set is a combined U.S. EPA survey and USGS data set ([data set] U.S. EPA 2016).

c

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

d

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