Identifying domestic well locations and populations served in the contiguous United States

Transcription

1 Identifying domestic well locations and populations served in the contiguous United States Tyler Johnson & Kenneth Belitz USGS National Water Quality Program

2 Background More than 44 million people in the United States rely upon selfsupplied water sources such as privately-owned domestic wells. More than 3.5 billion gallons of groundwater are withdrawn from domestic wells per day (Maupin et al., 2014). Domestic wells are often shallow and may be susceptible to contamination from the land surface or shallow subsurface.

3 Background shallow aquifers In 1991, Congress established the National Water Quality Program (NAWQA) 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. Given that location can affect water quality, it is important to define the location of domestic wells and the population that they serve This information along with information on groundwater quality can be used to help define risk

4 Background previous work, CA Statistical survey ~ 740,000 optically scanned Well Completion Reports (WCRs) Sampling at township scale (36 mi2) viewed & coded ~ 42,000 WCRs Census Tract Data, 1990 ~460,000 households served by domestic wells, ~5600 tracts Households on domestic wells Section scale (1 mi2) Johnson and Belitz, 2015, Identifying the location and population served by domestic wells in California, Journal of Hydrology: Regional Studies,

5 National-Scale Contiguous United States Block Group Method (BGM) Road Enhanced Method (REM)

6 Block Group Method US Census data o State > County > Tract > Block-group > Block Source of household water, including self-supplied Most recent data: 1990 Domestic-usage data is available at the block-group scale Population data is available at the block scale We downscaled the domestic-usage data and combined it with the population data

7 Result Block-Group Method

8 Block group method good but Areas of low density, especially in the western US Can we do better (increased spatial resolution)? Previous work in CA was based on statistical sampling of well completion reports WCRs are not available nationwide An alternative is the Road-Enhanced Method (REM) o Fundamental assumption is that houses (and domestic wells) are located near roads (named roads) o Buffer expansion and shrinkage technique

9 Road Enhanced Method: buffer expansion and shrinkage Roads Create a buffer # meters around the roads Shrink the buffer by # meters What is the appropriate buffer distance?

10 Question was answered using data from California Evaluate relationship between roads and wells Road network, wide range o Large multilane freeways to small backroads o Linear grid-based roads to winding byways Population served by domestic wells, diverse settings o Coastal, inland, mountain, desert o agricultural, rural, suburban, urban o Permanent and seasonal (eg. national parks and forests) Road network Domestic wells Road Enhanced Method: What is the appropriate buffer distance?

11 Question was answered using data from California Domestic well map is the calibration map Apply buffer and shrinkage procedure to the road network map o 21 buffer sizes ranging from 500m to 2500m o Each of the 21 REM maps were compared to the calibration map and evaluated for errors Underestimation REM map misses a section with domestic wells Overestimation REM map places wells into sections that do not have domestic wells Road network Domestic wells What is the appropriate buffer distance?

12 Underestimation Error, thousand km Type 1 error (underestimation) Road buffer distance, meters Small buffer leads to large Type 1 error

13 Underestimation versus overestimation Error, thousand km Type 2 error (overestimation) Type 1 error (underestimation) Road buffer distance, meters Small buffer leads to large Type 1 error Large buffer size leads to large Type 2 error Balance of errors at ~1 km

14 Evaluate the 21 REM maps: selected 1 km buffer Unbiased o The area underestimated is about equal to the area overestimated o Total area with wells is preserved (REM map ~ calibration map) Total error is minimized (underestimation + overestimation) What is the spatial accuracy? o Sections with wells that are missed are, on average, 1 km outside the area identified by the REM method (section is 1 mi 2 ) o Wells that are falsely identified are, on average, 2 km from a section with a well on the calibration map Less inclusive of wells than larger buffer sizes (especially the BGM)

15 Result BGM and REM Block Group Method Road-Enhanced Method

16 Result Colorado BGM REM

17 Result New England BGM REM

18 National maps are available Two spatial resolutions 1 km m x 100 m Block Group Method Johnson and Belitz, 2017, Domestic well locations and populations served in the contiguous U.S.: 1990, Science of the Total Environment Road Enhanced Method