Fig 1.
The exponential increase in U.S. frac sand production in the late 2000s.
The Midwest accounted for 68% of U.S. frac sand production in 2012, with Wisconsin accounting for the largest overall share. Texas was the leading frac sand producer outside of the Midwest. The dotted line indicates U.S. sand production for all end uses other than hydraulic fracturing. Data: U.S. Geological Survey [36].
Fig 2.
Map of silica sand deposits and probability of frac sand mine occurrence in Wisconsin, USA.
Probabilities were calculated using Maxent software [57] based on depth to sand, distance to major roads and rail lines, and land cover type. County boundaries and public lands are shown only for counties containing silica sand.
Table 1.
Mine counts and confidence intervals for neutral model predictions, by zoning and planning status for statewide and county-level analyses.
Fig 3.
Actual (A) and predicted (B and C) frac sand mine counts per county.
Predicted mine counts were based on two neutral landscape models at different spatial scales. Each predicted count is the average of 500 point permutations distributed based on probability of mining. Numbers outlined in red indicate actual mine counts that were higher than predicted confidence intervals in both neutral model analyses; numbers outlined in green indicate counts lower than predicted confidence intervals. Spatial extent (C) does not include the furthest northwest county because its mine was a spatial outlier.
Fig 4.
Four of the Wisconsin counties where frac sand mines clustered in unzoned jurisdictions.
In seven of the nine counties having some unzoned land and at least three frac sand mines, mines were more concentrated in unzoned areas than would be expected due to geological suitability and transportation proximity alone.
Table 2.
Stringency of county zoning ordinances and land-use plans in counties with higher- and lower-than-expected mine counts in county-zoned areas.
Table 3.
Relative log count of frac sand mines in Wisconsin townships as predicted by four generalized linear models.
Fig 5.
Predicted frac sand mine counts per Wisconsin township, plotted against distance to urbanized area.
Mine count estimates were calculated using a generalized linear model (Model 3). Dotted lines show separate predictions for 17 Wisconsin counties; solid lines show the mean prediction across all counties, by zoning status.