A New York City rally against fracking, Oct. 19, 2013. Photo: a katz, Shutterstock

The chemical cocktail used in unleashing natural gas pockets underground through the process called fracking appears to cause hormonal imbalance and fertility problems, according to a new study.

The research is among a growing list of studies contending the wastewater produced by the booming American industry could pose health problems to millions living near the deep wells.

A group of 23 gas and oil compounds were mixed in four different concentrations, and administered to a group of pregnant mice through drinking water, according to the study, published in the journal Endocrinology.

The animals’ development of ovarian follicles, and hormone levels from the pituitary and reproductive systems indicated reduced fertility, said the University of Missouri researchers.

The same team had previously linked reduced sperm counts in male mice to the chemicals.

“Evidence from this study indicates that developmental exposure to fracking and drilling chemicals may post a threat to fertility in animals and potentially people,” said Susan Nagel, associate professor of obstetrics, gynecology, and women’s health, and lead author of the paper. “Negative outcomes were observed even in mice exposed to the lowest dose of chemicals, which was lower than the concentrations found in groundwater at some locations with past oil and gas wastewater spills.”

The mice showed lower levels of hormones needed for healthy reproductive processes, they concluded.

A crowdfunding campaign partly funded the research.

Fracking is a drilling method to capture natural gas deposits deep beneath ground. During the process a mix of water, sand and chemicals is pumped more than a mile underground, causing explosions that unlock the pockets of natural gas. Several studies have pointed to environmental dangers, and it has been the source of political controversy from national to local venues.

Industry groups say the technique is safe and could unleash the energy equivalent of 87 billion barrels of oil underneath the Marcellus Shale, an arc of underground rock from West Virginia and Ohio up to Pennsylvania and New York.

The natural gas industry says the resource is the nation’s clean alternative to coal and oil and the chemicals used are too deep to affect the water table. Chemicals make up less than one percent of the mixture injected underground, the industry contends – though the exact composition of that cocktail has been kept an industry secret.

Energy in Depth, an industry group, has blasted Nagel’s work before. Katie Brown, a scientist working for the group, wrote an op-ed for the group Thursday arguing that the Missouri experiments were rigged to show problems.

“Let’s just say if you give pregnant mice only contaminated water to drink – and at extremely high concentrations – is it any surprise that they might have abnormalities as a result?” Brown writes. “In other words, the researchers concocted the most unlikely scenario – continuous exposure to chemicals at high concentrations – and then tried to pass it off as plausible.”

Other studies have linked health problems to fracking areas, including increased hospitalization rates.