The amount of water required by producers to fracture oil and natural gas wells varies across the country, but one thing is certain — the annual volumes used are increasing dramatically, according to the first nationwide map of usage by the U.S. Geological Survey (USGS).

Within watersheds across the United States, the average amount of water used for fracturing ranged from as little as 2,600 gallons/well to as much as 9.7 million gallons/well. The information compiled for the Department of Interior agency using IHS Inc. data on 81,000 wells across the country.

“One of the most important things we found was that the amount of water used per well varies quite a bit, even within a single oil and gas basin,” said lead author and USGS scientist Tanya Gallegos. “This is important for land and resource managers, because a better understanding of the volumes of water injected for hydraulic fracturing could be a key to understanding the potential for some environmental impacts.”

The watersheds where the most water was used to fracture wells are within most of the biggest shale plays in the United States:

Shale gas reservoirs are often fractured using slickwater, a fluid type that requires a lot of water. In contrast, tight oil formations like the Bakken Shale in parts of Montana and North Dakota often use gel-based treatment fluids, which generally contain lower amounts of water, USGS noted. The data does not cover all fractured wells in the United States as some states do not require companies to report how much water they use in the drilling process.

From 2000 to 2014, median annual water volume estimates for fracturing horizontal wells jumped dramatically, from about 177,000 gallons per oil and natural gas well to more than 4 million gallons for each oil well and 5.1 million gallons per gas well. Meanwhile, median water use in vertical and directional wells remained below 671,000 gallons per well. For comparison, an Olympic-sized swimming pool holds about 660,000 gallons of water.

In 52 out of the 57 watersheds with the highest average water use for fracturing, more than 90% of the wells were horizontally drilled, USGS noted.

Horizontal wells first are drilled vertically or directionally, at an angle from straight down, to reach the unconventional oil or gas reservoir and then laterally along the oil or gas-bearing rock layers to increase the contact area and stimulate more production than could be achieved through vertical wells alone. However, horizontals also generally require more water than vertical or directional wells.

A huge increase in the number of horizontal wells drilled has happened since the shale revolution began in 2008. However, in 2014, about 42% of the wells completed still were vertical or directional.

“The ubiquity of the lower-water-use vertical and directional wells explains, in part, why the amount of water used per well is so variable across the United States,” said the authors.

The research, which provides the first national-scale analysis and map of fracturing water usage, is to be published in American Geophysical Union’s journal Water Resources Research. Thestudy, “Hydraulic fracturing water use variability in the United States and potential environmental implications,” is part of a larger effort by the USGS to understand resource requirements and potential environmental impacts of unconventional oil and gas development.