In his draft report to the Shelter Island Irrigation Committee, consultant John Benvegna isn’t recommending an outright ban on existing automatic irrigation systems on Shelter Island.
But he is suggesting revisions to watering schedules that would reduce the stress on wells and the potential for saltwater intrusion.
While Mr. Benvegna approved the public release of his draft report, he indicated that it doesn’t include data from the most recent June field tests. At the same time, he said he doesn’t expect the new data will change his basic recommendations. But he wanted to keep the door open to some changes he could make by including that data, expected at the end of June.
“Based on the available information, there is no evidence that operation of permitted irrigation systems, in accordance with the current irrigation regulations, has adversely impacted groundwater resources on Shelter Island,” said the consultant from the White Plains office of the Connecticut-based Leggette, Brashears & Graham. Only 1.2 percent of average annual recharge is used for consumptive purposes, prompting Mr. Benvegna to conclude, “There are sufficient water resources available to support the water supply demands of Shelter Island.”
His evidence is based not only on field studies completed by his team in the past several months and well water level readings conducted by the United States Geological Survey, but on studies conducted by various experts in the last 40 years.
His and other studies show that with an estimated year-round population of 2,500 and summer population of 10,000, water use represents only about 5 percent of the average annual recharge.
Why, then, should the Irrigation Committee and Town Board consider a ban on irrigation systems as contained in legislation passed in 2003 that was originally to be implemented in 2013?
That answer varies depending on which members of the Irrigation Committee are speaking.
John Hallman, who also is chairman of the town’s Water Advisory Committee, worries that it’s inevitable that problems he observes in some parts of the Island will expand in those areas unless major conservation methods are undertaken. Committee member Walter Richards, who previously served on the Water Advisory Committee, favors ongoing monitoring of conditions, but isn’t so sure any ban is needed.
And everyone has concerns that the existing 109 registered irrigation systems on the Island, along with perhaps 50 to 100 more that aren’t registered, could proliferate, especially in areas where there are already reported problems like the Rams.
That’s why the committee is expected to recommend regulations that would apply to low lying areas — the Rams, Silver Beach, Shorewood — but not to the Center or Shelter Island Heights (See separate Irrigation Committee story).
One of the least understood facts about Shelter Island, Mr. Benvegna said in his draft report is how water flows — or doesn’t flow — from one Island area to another.
True, there is a single aquifer, but it’s not like one large bowl from which all parts of the Island can draw as needed. Because of geological rock formations, the existence of a thick clay level and various waterways around the Island, water is never going to flow from the Center to the Rams, for example, Mr. Benvegna said. Borings show that clays are at least 200 feet thick beneath most of the Island creating what he describes as “like a layer cake.”
“Fresh groundwater only occurs within the upper glacial aquifer,” he said, explaining that sampling shows that water in the marine clay and underlying formations is “brakish to saline and that concentrations increase with depth.”
That freshwater-saltwater zone extends up into the upper glacial aquifer where it limits the vertical flow of freshwater in some areas, according to the report. The horizontal flow of freshwater is limited by inland waterways, including Coecles Harbor, West Neck Bay and West Neck Harbor, That results in less fresh water beneath peninsulas such as the Rams and Silver Beach, he said.
Mr. Benvegna’s conclusions are backed up by previous reports he reviewed that mirrored his own team’s observations.
There are seasonal fluctuations in freshwater availability that reflect precipitation amounts and those fluctuations have remained consistent through the years, the report shows. Annual precipitation averages 48 inches of which about half becomes recharge into the aquifer. The rest either flows into waterways or is absorbed into plants through transpiration and then evaporation.
In addition to staggering watering times for those with irrigation systems, Mr. Benvegna is making the following recommendations to the town:
• Institution of regular ongoing chloride monitoring, something that was done by Mr. Benvegna’s field test team, but isn’t done regularly by the United States Geological Survey’s monthly well monitoring that only looks at water levels.
• Creation of several additional monitoring wells at the bottom of the upper glacial aquifer to provide better data on the diffusion zone boundary. Existing well monitoring is of wells that don’t extend more than 45 feet below sea level and the majority are at 30 feet below sea level. At the bottom of the upper glacial aquifer, where increasing chlorides would first be detected the depths are between 50 and 100 feet below sea level.
• An investigation of the possible use of gray water (treated effluent) from the Shelter Island Heights wastewater treatment plant to fill cistern systems used for irrigation. Currently the Heights treatment plans has a 53,000 gallons per day discharge limit and the discharge goes into the water near North Ferry.
Since treated effluent still contains nitrates that the State Department of Environmental Conservation has concluded cause algae blooms and pose other problems for the water, the use of that effluent in cisterns meant to feed irrigation systems would not only protect the waterways, but reduce the costs of cistern operation. Currently, the 14 registered cisterns and an unidentified number of unregistered cisterns, are required to be filled with expensive trucked-in water.
Using the gray water would reduce costs, inducing more people to use cisterns, while affording greater protection of surrounding waterways.