Because of increased natural gas exploration throughout the Marcellus Shale Fairway, many water systems are developing source water protection plans to help manage the potential threat to the quality and quantity of their drinking water. An example of this potential issue comes from a small water system located in northcentral Pennsylvania .
This block diagram shows the relationship between the gas-bearing rock formation (in orange) with the overlying fresh water aquifer (in blue).
An improperly constructed top hole of an abandoned gas well provides a potential migration path between the two rock formations.
The elevation data displayed on the sides of the block are expressed in feet, relative to mean sea level.
The South Renovo Borough Water System (SRBWS) provides drinking water to approximately 540 people in Clinton County. South Renovo receives its water from two sources: a surface water reservoir and a groundwater well that supplements the reservoir during droughts. Recently, the system began experiencing operational problems due to the presence of methane discovered in the water drawn from the groundwater well. The water system brought this contamination problem to the attention of the PA Department of Conservation and Natural Resources (DCNR). The DCNR identified a gas well, located 2,500 feet north of the groundwater well, as a possible source of the methane migration. According to the DCNR, the old gas well was drilled in 1953, and was plugged and abandoned that same year. The materials and techniques used to seal the gas well are over 50 years old and may be of questionable integrity. The DCNR theorized that the methane leaking from this old well is migrating from the gas-bearing rock, up through the compromised top hole and into the fresh water aquifer feeding the SRBWS groundwater well.
In 2009, SRBWS entered the Department of Environmental Protection’s (DEP) Source Water Protection Technical Assistance Program. The objective of this program is to develop a source water protection plan that delineates the recharge areas for water sources, identifies potential sources of contamination, develops water protection management strategies, and complies with the DEP’s Chapter 109 regulations. As part of the delineation process, SSM identified the recharge area for both the groundwater and surface sources for the Borough’s Halls Run watershed and recommended locations for a replacement well.
South Renovo Water System sources (in red), the source water protection zones, the Halls Run watershed, natural gas well locations (in yellow), and the location of the abandoned gas well (in white).
The DCNR is continuing to examine the South Renovo case to definitively determine the source of methane in the groundwater well. Geochemical analysis, or “fingerprinting,” is widely used to identify the source of stray gas migration within natural gas fields. Regardless of the outcome of the case, the DCNR raises an interesting point: a source of groundwater contamination can exist, not only outside the protection zone, but down-gradient from the water source. This concept defies conventional wisdom that water flows downhill; therefore, pollution also flows downhill. This is simply untrue when dealing with stray gas migration issues. Natural gas, being less dense than water, can migrate, from depth to the shallow aquifer layers. This fact emphasizes the point that groundwater protection requires a three-dimensional approach. Working with water systems throughout the Commonwealth, SSM Group creates three-dimensional groundwater flow computer models to assist the water system in understanding how water moves through their aquifer system.
Source: SSM GROUP INC. | Engineering and Environmental Services | www.ssmgroup.com