Fenwick Falls Plans Ahead: A Buried Valley and a Gas Dome

Meeting Notes: Where Will Our Water and Heat Come From?

1Fenwick Falls is a city of about 40,000 people in the northern United States. For a hundred years it has taken its drinking water from the Fenwick River, but in dry summers the river runs low. The city planning board is looking for a second water source, and it also wants to know whether natural gas found under the eastern hills could heat city buildings. At a public meeting, geologist Dr. Marisol Ventresca explained what lies under the city. Anouk Fairweather, a seventh grader on the youth advisory board, took notes.

2About 15,000 years ago, a sheet of ice more than a kilometer thick covered this region. As the climate warmed, the ice melted back. Rivers of meltwater rushing out from under the ice carved deep, narrow valleys into the bedrock and filled them with sand and gravel. Elsewhere, the melting ice dropped till, a tightly packed mix of clay, sand and stones. Till now covers the whole area, so the land around Fenwick Falls looks flat and gives few clues about what lies below.

3Groundwater fills the spaces between grains of sediment and the cracks in rock. Water flows easily through the large, connected spaces in sand and gravel. In till, clay fills the spaces between the stones, so water moves through it very slowly. The bedrock under the city is shale, which has tiny, poorly connected spaces. The city drilled five test holes along a line from west to east across the city (Table 1).

4Under the eastern hills, the rock layers are bent upward into a dome, shaped like an upside-down bowl. One layer in the dome is sandstone, and a thick layer of shale lies on top of it. Long ago, the remains of tiny sea organisms were buried in mud, and over millions of years heat and pressure slowly changed them into oil and natural gas. Oil, gas and salty water then moved through the connected spaces in the sandstone. Gas is less dense than oil, and oil is less dense than water. A company drilled five test wells into the sandstone layer (Table 2).

5Dr. Ventresca also talked about limits. Rain and snowmelt soak down through the till and refill the buried sand and gravel with about 6 million liters of water a day on average. The board's plan would pump 9 million liters a day from new wells. The company estimates that the gas in the dome would heat the city's buildings for about 30 years.