The Potential Influence of Frac Sand Mining on Above and Belowground Ecosystem Productivity.

Wisconsin is currently the primary producer of high quality frac sands (i.e., Silicon Dioxide (SiO₂)) integral to the hydraulic fracturing process as the proppant that keeps shale fractures and pores open during the hydrocarbon extraction process. This use is coming to rival – and at times superseding in some years – silica sand’s historical uses. The influence of frac sands on the stocks/flows and pricing of silica has entered the exponential – and heretofore undocumented – phase of its growth trajectory. The directional drilling industry has moved away from historically large but relatively heterogeneous and angular stockpiles in Texas and Oklahoma’s Riley and Hickory (Brady) formations. The Great Lakes sands derived from the unconsolidated alluvial and poorly cemented Cambrian and Ordovician St. Peter (Ottawa) Sandstone Formation are more reliable, crush resistant, pure, and spheroid. Wisconsin’s mines (n = 24, 19% of currently operating mines) are producing 0.69-2.50 million tons per mine or 4,087-9,397 tons per acre. The West Central Wisconsin region is producing 211-336 million silica sand tons equivalent to 93,777-149,333 horizontal wells.

In an effort to quantify total above and belowground carbon dynamics we conducted land cover analysis across Wisconsin’s 1,031 sandstone polygons with a total area of 60,761 km². Forest and cultivated lands constitute the largest fraction of Wisconsin’s sandstone geology with respective values of 21,542 and 28,349 km². The aforementioned forests generate 10.1-21.8 million tons of woody biomass per annum, 2.3 million tons of herbaceous carbon, 11.0-23.8 million tons of foliage, 614,477-1,331,247 tons of roots, and 140,444-303,400 tons of arbuscular mycorrhiza. Put another way productivity generates 72.7-113.6 million m² of basal area. Turnover of the above generates 4.5-9.8 million and 9,646-20,898 tons of soil carbon and nitrogen. The latter has a fertilizer equivalent market value of $5.0-10.7 million. According to our analysis these sandstone polygons generate 27.8 million tons of crop biomass per year worth an estimated $4.9 billion.  Finally, we conducted similar analysis for Minnesota and found that the Minnesota sandstone polygons supporting forests generate 4.1-4.9 million tons of woody biomass and 11.3-17.0 million m² worth of basal area.

As Wisconsin – and Minnesota – considers the title of “Silica Arabia” bestowed upon it by industry journalists it will be important for regional planning commissions, state political figures, and regulatory agencies to consider the complete suite of costs and benefits associated with frac sand mining, reclamation best practices, forest fragmentation, and watershed dynamics integral to ecological resilience. These considerations will likely include the incorporation of the above estimates in ex ante and ex post decision making trajectories.