Facilities that compost vegetation appear to cause contaminants to leach into groundwater and should be more tightly regulated by the state, according to a new Suffolk County report.

The state said it plans to introduce new controls over such facilities and added that the county’s findings would have been better supported if it had done more testing by installing more wells.

The Jan. 22 report by the county’s Department of Health Services relies on results from 233 groundwater samples and two surface-water samples taken from 36 wells adjacent to 11 facilities in the county that either currently compost vegetative waste or once did so.

It found elevated levels of metals, particularly manganese, in groundwater — in one case, as high as 160 times the drinking water and groundwater standards. Other metals with elevated levels included thallium and iron.

Manganese and other elements can leach into the soil when organic material decomposes, according to the state Department of Environmental Conservation.

At low levels, manganese is an essential part of the human diet, and the World Health Organization found manganese “is not considered to be very toxic when ingested.”

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Overall, the report found the metals were at levels higher than what is typically found in shallow private wells in the county.

The sampling was performed between July 2011 and October 2014, mostly in spots where the groundwater flowed away from the composting facilities.

Four of the facilities were in Manorville, two were in Yaphank, and one each was in Speonk, Ronkonkoma, Eastport, East Farmingdale and Medford.

The report concluded that groundwater was impacted by the composting facilities at eight of the sites, while impacts from the other three could not be determined.

The county study was prompted after elevated levels of contaminants previously were found in a private drinking-water well and several monitoring wells near Great Gardens/Long Island Compost in Yaphank.

The DEC issued a 2013 report on that facility in cooperation with the state and county health departments, finding that the site “appears to be the primary source of the contamination” and recommending testing other sites.

Long Island Compost had agreed to change its practices to reduce the impacts to groundwater from its operations as a result of the investigation, the DEC said.

The highest manganese levels in the county’s January report were found at a site on East Main Street in Yaphank. A groundwater sample there measured 49,300 parts per billion of the metal — higher than the 31,600 parts per billion found earlier in the private well near Great Gardens.

While groundwater is the source of Long Island’s drinking water, the report said no wellfields that supply drinking water to the public were “imminently threatened.”

The Suffolk County Water Authority “has concerns with any activity that negatively impacts groundwater and supports measures that will continue to protect our sole-source aquifer,” Ty Fuller, the authority’s lead hydrogeologist, said in a statement.

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Groundwater on Long Island is treated to meet standards before it is delivered to the public.

The study recommended that the DEC more strictly regulate mulch facilities and evaluate any potential impacts to groundwater before allowing such facilities to operate.

The DEC responded to a draft of the report in a Dec. 11 letter to Dr. James Tomarken, commissioner of the county’s health department.

In that letter, the DEC said it planned to introduce “regulatory criteria for mulch-processing facilities that are currently exempt from regulation” in the state’s forthcoming Part 360 solid-waste regulations, due to be released at the end of this month.

Under the proposed regulations, those facilities would be required to manage runoff, and larger facilities could be required to install monitoring wells and test ground and surface water.

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Before the new regulations go into effect, the DEC said it would “evaluate the larger sites and determine what mitigation measures could be used in the cases where the Department has regulatory control over the facilities.”

Mulching facilities have been a subject of concern for environmentalists and legislators, including state Assemb. Steve Englebright (D-Setauket), who last year authored a bill that would have imposed stricter environmental regulations on mulch facilities in Nassau and Suffolk counties.

“I think the report confirms the seriousness of this issue and underscores the need for regulatory oversight,” said Englebright, who is chair of the Assembly’s Environmental Conservation Committee.

But the bill was vetoed by Gov. Andrew M. Cuomo, who said the legislation was unnecessary because the state planned to release the new draft regulations.

In its response to the report, the DEC also said the county should have tested groundwater leading into the sites as well as flowing away from them, adding that the lack of such testing “makes it difficult to determine the contribution” by such facilities to the elevated levels of contamination.

Grace Kelly-McGovern, spokeswoman for the county health department, said in a statement that the county did not install the additional wells because the study was not designed for regulatory or enforcement purposes.

“The results are clear and convincing,” she said.