National Water-Quality Assessment (NAWQA): 1991-2012

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In 1991, Congress established the National Water-Quality Assessment (NAWQA) Project to address where, when, why, and how the Nation's water quality has changed, or is likely to change in the future, in response to human activities and natural factors. This page discusses the first two decadal cycles of NAWQA research from 1991 through 2012.

►  Read below about the first two decades of NAWQA research:

  • First decade - Baseline survey of water-quality conditions in 51 river basins and aquifers
  • Second decade - Synthesis reports on major water quality topics of national priority

►  Explore current NAWQA research


The NAWQA Project was designed to run in decadal cycles.  From 1991-2001, the first decade, the NAWQA Project conducted interdisciplinary assessments and established a baseline understanding of water-quality conditions in 51 of the Nation's river basins and aquifers, referred to as Study Units. Descriptions of water-quality conditions in streams and groundwater were developed in more than 1,500 reports. Non-technical Summary Reports, written primarily for those interested or involved in resource management, conservation, regulation, and policymaking and listed below, were completed for each of the Study Units. Non-technical national summary reports on pesticides, nutrients, trace elements, and volatile organic compounds (VOCs) also were completed, in which water-quality conditions were compared to national standards and guidelines related to drinking water, protection of aquatic life, and nutrient enrichment.

NAWQA activities during the second decade (2002-2012) focused in large part on national and regional assessments. These assessments built on continued monitoring and assessments in 42 of the 51 Study Units completed in the first decade (USGS Fact Sheet 071-01). During the second decade, there was an emphasis on synthesizing the results of the Study Unit investigations on major water-quality issues:

New studies during the second decade focused on water-quality topics of national priority.

►  Additional information on these and other water-quality topics


NAWQA Study Units

NAWQA Study Units covered a variety of important hydrologic and ecological resources; critical sources of contaminants, including agricultural, urban, and natural sources; and a high percentage of population served by municipal water supply and irrigated agriculture. Study Unit boundaries frequently croseds State boundaries and usually encompassed more than 10,000 square kilometers (about 3,900 square miles).

The Study-Unit design used a rotational sampling scheme; therefore, sampling intensity varied year to year at the different sites. In general, about one-third of the Study Units were intensively investigated at any given time for 3-4 years, followed by low-intensity monitoring. Trends in water-quality conditions were assessed about every 10 years.

NAWQA study units, 1991 - 2004

Summary reports are listed below for the individual Study Units assessed during the first decade (1991-2001).  Publications for principle aquifer assessments are listed at Groundwater Quality in Principal Aquifers of the Nation, 1991–2010.


Study Unit Summary Reports

(Number refers to the study units map, above)

1 - Acadian-Pontchartrain Drainages (LA, MS)

2 - Albemarle-Pamlico Drainage Basin (NC, VA)

3 - Allegheny and Monongahela River Basins (MD, NY, PA, WV)

4 - Apalachicola-Chattahoochee-Flint River Basin (AL, FL, GA) 

5 - Central Arizona Basins (AZ) 

6 - Central Columbia Plateau (ID, WA)

7 - Central Nebraska Basins (NE) 

8 - Connecticut, Housatonic and Thames River Basins (CT, MA, NH, NY, RI, VT)

9 - Cook Inlet Basin (AK)  

10 - Delaware River Basin (PA, NY, NJ)

11 - Delmarva Peninsula (DE, MD, VA)

12 - Eastern Iowa Basins (IA, MN)

13 - Georgia-Florida Coastal Plain (GA, FL)

14 - Great and Little Miami River Basins (OH, IN)

15 - Great Salt Lake Basins (UT, ID, WY) 

16 - Hudson River Basin (NY, CT, MA, NJ, VT) 

17 - Island of Oahu (HI)

18 - Kanawha-New River Basins (WV, VA, NC)

19 - Lake Erie-Lake Saint Clair Drainages (IN, MI, OH, PA, NY)

20 - Long Island-New Jersey Coastal Drainages (NJ, NY) 

21 - Lower Illinois River Basin (IL)

22 - Lower Susquehanna River Basin (MD, PA) 

23 - Lower Tennessee River Basin (TN, AL, GA)

24 - Las Vegas Valley Area and the Carson and Truckee River Basins (NV, CA) 

25 - Mississippi Embayment (AR, KY, LA, MS, MO, TN)

26 - Mobile River Basin (MS, AL, GA)

27 - New England Coastal Basins (ME, MA, NH, RI)

28 - Northern Rockies Intermontane Basins (ID, MT, WA)

29 - Ozark Plateaus (AR, KS, MO, OK)  

30 - Potomac River Basin (DC, MD, PA, VA, WV)

31 - Puget Sound Basin (WA) 

32 - Red River of the North Basin (MN, ND, SD) 

33 - Rio Grande Valley (CO, NM, TX)

34 - Sacramento River Basin (CA)

35 - San Joaquin-Tulare Basins (CA)

36 - Santa Ana Basin (CA)

37 - Santee River Basin and Coastal Drainages (SC, NC)

38 - South-Central Texas (TX)

39 - South Platte River Basin (CO, NE, WY) 

40 - Southern Florida (FL)

41 - Trinity River Basin (TX) 

42 - Upper Colorado River Basin (CO, UT) 

43 - Upper Illinois River Basin (IL, WI, IN)

44 - Upper Mississippi River Basin (MN, WI)

45 - Upper Snake River Basin (ID, MT, NV, UT, WY) 

46 - Upper Tennessee River Basin (GA, KY, NC, SC, TN, VA)

47 - Western Lake Michigan Drainages (MI, WI) 

48 - White River Basin (IN)

49 - Willamette Basin (OR)

50 - Yakima River Basin (WA) 

51 - Yellowstone River Basin (MT, WY, ND)