From NY DEP release from 7-29-15:
July 29, 2015
DEPARTMENT OF ENVIRONMENTAL PROTECTION PROVIDES
UPDATE ON REPAIRS AND MONITORING AT CANNONSVILLE DAM
The New York City Department of Environmental Protection (DEP) today provided the following update on the ongoing monitoring and repair efforts related a turbid discharge below Cannonsville Dam.
Late last week, construction crews and geotechnical engineers arrived at the site to begin repairs below the dam. The first step involves drilling a series of relief wells that will provide an
alternate path for the groundwater that’s under pressure, and prevent that water from moving up through the original bore holes. These relief wells will also end the mobilization of fine sediments that are causing turbidity in the West Branch Delaware River. The relief wells will be cased, screened and filtered to ensure only clear water is pumped from them.
As of Wednesday, four of these relief wells were at some stage of production. One relief well was finished and pumping, and two more were expected to be pumping within the next 24 hours. Engineers at the site noted a reduction in the turbid flow from the rock embankment once the first well began pumping – a sign that it had successfully tapped into the pressurized groundwater. The flow from the rock embankment below the dam is monitored around the clock and measured on a set of staff gauges, or large rulers, that were installed in that area. Engineers are prepared to install as many as 8-10 relief wells, but they expect it may take fewer to end the turbid discharge.
Once the relief wells are installed, engineers will then seal shut the original bore holes using a technique known as compaction grouting. This involves injecting high-pressure grout around the circumference of the original bore holes until they are completely sealed off from the underlying ground water. Engineers believe all this work could be done in the next several weeks.
In the meantime, DEP has continued its 24-hour monitoring of the dam, the worksite, and the flow from the rock embankment.
Over the past several days, two rounds of testing have confirmed that sediment reaching the West Branch Delaware River is coming from the immediate area around the original bore holes, and not from the earthen embankment of the dam itself. The tests were performed at an independent lab that specializes in material testing. To perform these tests, water samples were taken from the area of turbid discharge, and the sediment was screened out of the water. That sediment was then compared to samples taken from different depths along the original bore holes. The testing confirmed that sediments reaching the river are identical to very fine silts
located 25-35 feet below the rock embankment, in the immediate area of the original bore holes. This is not the material that comprises the dam.
As noted previously, DEP has also observed a reduction in the flow from the rock embankment area since the pumping of the first relief well began. The size of the silt particles creating the turbidity has not changed. All other metrics and monitoring points remain within
normal and expected ranges.
DEP also continues to monitor the storage of cold water in Cannonsville Reservoir. This cold water is vital to the downstream fishery in the West Branch and main stem of the Delaware River. There is approximately 32 billion gallons of cold water remaining in the reservoir. That cold water would be expended about Aug. 17. Once the relief wells end the turbid discharge below the dam, DEP plans to extend the life of that cold water by taking more of its drinking water from the warmer top layer of the reservoir. This operational change requires a two-hour shutdown of the drinking water intake. DEP expects to make that change sometime in the next several days.
DEP has also continued to provide the public with information as the work progresses. On Tuesday, DEP posted photos of the worksite, including an aerial image. These photos can be
found on the DEP’s watershed Facebook page at www.Facebook.com/NYCWatershed, or on the
Cannonsville Reservoir page by clicking here. Daily updates related to Cannonsville Reservoir storage and releases are also posted on that website.
