Subsidence forces shift to surface water

Subsidence forces shift to surface water

By Kimberly Stauffer Friday, 22 January 2010

HOUSTON — Houston residents can expect to see their water bills increase as the city and its outlying areas convert from groundwater to surface water this year.

The necessary conversion stems from long-term subsidence, or sinking, in the region caused by the overpumping of the Gulf Coast Aquifer that stretches from Florida to Mexico and supplies 54 Texas counties with all or part of their water supply. The Harris-Galveston Subsidence District, formed in 1975 by the state Legislature, was created to counteract as much as 10 feet of recorded subsidence in the two counties.

“Immediately upon creation of the district, we started doing studies to determine the exact relationship between groundwater pumpage and subsidence,” said Tom Michel, deputy general manager of the HGSD. “When we have an increase in water pumpage, it decreases the aquifer levels. Once [they have] decreased, if you continue with pumpage, subsidence will occur.”

Harris and Galveston county areas have experienced at least 1 foot of subsidence, with spikes as high as 13 feet. While northwest Houston has been unaffected in the past, Michel said new GPS monitoring sites at FM 1960 and Hwy. 290 have recorded one-fifth of a foot of subsidence each year, and the area is fast approaching 1.5 feet of subsidence.

“As the population has grown, generally towards the north and west areas, the demand for groundwater has increased. Subsidence is simply following the population,” he said. “As we convert from groundwater to surface water, subsidence has dramatically slowed or stopped.”

HGSD’s solution is to shift from groundwater to surface water. The district’s 1999 regulatory plan mandated the city convert 30 percent of groundwater usage to an alternate supply by 2010. Michel said the district believes all areas will be in compliance by the end of the year. In 2020, the district mandates areas must convert to 70 percent surface water, and by 2030, at least 80 percent of the water supply must come from the alternate source.

Regional conversion

Wayne Ahrens, engineer for the West Harris County Regional Water Authority, established with the HGSD’s 1999 regulatory plan, said approximately 51 miles of pipeline has been laid to meet the 2010 deadline throughout the authority’s 226-square-mile area, which stretches from south of Hwy. 290 to Clay Road, bordering Houston’s city limits to the east, Waller County line to the west and the Fort Bend County line to the south. Currently, the authority delivers surface water to 31 utility districts using 39 water plants, serving the Copperfield area, The Colonies, Hearthstone and the Remington communities. The WHCRWA is already using 25 percent surface water in its jurisdiction.

“The first plant we converted was in September 2005,” Ahrens said. “We’ve been converting slowly since then and have just a few left to convert for the 2010 system. All the pipelines for the 2010 system have been installed and are capable of delivering water at this time.”

WHCRWA receives 28.25 million gallons of surface water daily from the City of Houston’s Jersey Village Water Plant through a contract with the city. The need will increase for the 2020 phase, Ahrens said, and the city cannot supply any more water than it is now. It will force the authority to use the city’s Northeast Water Purification Plant for an additional 50 million gallons per day to meet the 70 percent surface water use mandate.

On the other side of Hwy. 290, the North Harris County Regional Water Authority has laid almost 70 miles of pipeline to begin delivering surface water to 64 communities, some of which should begin receiving surface water this month from the San Jacinto River System. The authority began constructing its system in 2005 because many utility districts’ wells were pumping out water with high amounts of impurities, especially arsenic, said Al Rendl, a member of NHCRWA’s board of directors. NHCRWA used the pipelines to transfer water from districts with excess water to districts in need.

A sticking point for both authorities has been the conversion from chlorine to chloramines for disinfecting groundwater, Rendl said. While chlorine easily cleans groundwater, chloramines are needed to cleanse surface water. The two chemicals do not mix, forcing the authorities to convert all purification systems before they can begin pumping in surface water.

Invisible infrastructure

For any city or community to exist and grow, water is tantamount, Michel said.

“Water is the invisible infrastructure,” he said. “For the most part it’s buried underground.”

While agriculture and single-family wells are exempt from the regulatory plan, the rest of the HGSD area will pay the price for surface water—literally.

“There will definitely be a price increase. Conversion is not cheap,” Michel said. “The price will double to as much as three or four times the rate for groundwater.”

Water rate increases are an ongoing issue. When the regulatory plan was adopted in 1999 and the water authorities created, the price of water immediately rose, Michel said. The price went up again in the early 2000s.

Water rates have increased annually for WHCRWA residents as the authority continues pipeline construction. Ahrens said prices were also impacted by the economy, bond market and increased general building costs. As of Jan. 1, 2010, the WHCRWA groundwater pumpage fee increased from 95 cents to $1.25 per 1,000 gallons and residents using surface water saw a rise from $1.25 to $1.55 per 1,000 gallons.

“[Residents using surface water] are paying more to make the cost of water equal,” Ahrens said. “Generally, the rates have been going up each year. We anticipate an increase for the next number of years.”

The NWCRWA residents asked the board to keep costs low for as long as possible, Rendl said. The groundwater pumpage fee has risen from 25 cents in 2000 to $1.50 in 2005. Now, effective Jan. 1, 2010, the NWRWA groundwater pumpage fee is $1.75 per 1,000 gallons and surface water users pay $2.20 per 1,000 gallons. Rendl said the 45-cent discrepancy accounts for the cost and effort involved in pumping and purifying groundwater.

“Will that increase in the future? Yes, it will,” Rendl said. “We will have to increase the price of water to pay off our bonds. We have a huge project in front of us.”

A main reason groundwater continues to cost less per 1,000 gallons is area aquifers supply high quality water that requires less purification and is more easily available than surface water, Michel said.

“To get surface water, you need to build big pipelines and bring water back,” he said. “That’s where the big cost is, in transportation.”

Surface water requires more treatment, filtering out heavy debris and sediments from lakes and rivers. Michel said he expects to see more regionalization of the water treatment process in the future, rather than small plants in every MUD or city.

Urban sprawl

While pumping water from the Gulf Coast Aquifer is an issue, urban development is one of the main causes of subsidence.

“It’s a story we are going to see in urban areas repeated again and again,” said Jeanne McDonald, attorney for MUD 168. “When you go to a cow pasture, it’s all dirt. Water hits the dirt and goes down deep to replenish the aquifer. Every time you build a house or a driveway or a street, you cover the earth with cement, reducing the amount of dirt with impervious cover.”

While new developments continue to drill wells and pump out water, no new rainwater can seep into the depleted aquifer.

“It’s killing off the aquifer’s circle of life,” she said. “That’s what land development does for aquifers.”

Geology

Fred Garcia, director of communications for the Harris County Flood Control District, said the area’s geology is the deciding factor in why the area needs to convert to surface water from groundwater.

“The underlying geology of our region is when water is taken out and the pressure is taken out of the subsurface, our geology is such that it compresses and collapses as opposed to Austin or San Antonio, where they are built on bedrock and caverns,” Garcia said. “They can afford to have water and pressure taken out. Our geology dictates why we have to make the conversion.”

Even though the ongoing drought has dissipated area lakes, HGSD’s Michel said the greater Houston area has an adequate water supply for “many, many decades.” His concern is more for people understanding the value of water.

“I really hope nobody ever has to worry about water. We all take our water supplies for granted,” he said. “The state did a poll on where folks get their water; the vast majority response was from the tap. They really had no clue where they get their water. It’s good that we’ve done such a great job as water planners that we have a good supply of quality water, but at the same time nobody sees the value of water and what happens when you don’t have it.”

Understanding subsidence

• Subsidence is the sinking of land after too much groundwater has been pumped out to supply local developments. As part of the Gulf Coast Aquifer, Houston is near sea level and built on layers of sand and clay, unlike other aquifers, such as the Edwards Aquifer in Central Texas, which rests on layers of bedrock.
• The Harris-Galveston Subsidence District was created in 1975 in response to this issue. Formed by the state Legislature, the agency works to counteract as much as 10 feet of recorded subsidence in the two counties.
• Converting to surface water will allow groundwater to replenish and prevent further subsidence throughout the Houston area.