Red Arroyo Project Could Provide a Third of City's Annual Water Needs

Members of the Upper Colorado River Authority (UCRA) and the City Water Department presented Council with an update on the Red Arroyo Feasibility Study Tuesday, delivering information on projected water savings, costs of the project, and potential areas of difficulty if the plan is to be implemented.

The UCRA began monitoring storm water back in 2010 via 10 permanent stations set up along the two forks of the Arroyo and at the point where they meet. By studying historical averages of annual rainfall and measuring storm water flow through the Red Arroyo Channels, the UCRA determined that an estimated 500 acre-feet of storm water per inch of rainfall could be collected, with an average of 20 inches per year hitting San Angelo.

“These stations were set up, and through that we started realizing—we all knew that there was a lot of water that flows down the Red Arroyo, but we never really quantified that,” says Chuck Brown, Director of Operations for the UCRA. “In these stations we were able to actually quantify that amount of water.” Having looked at the numbers, “we said, ‘wow, we’ve got 10,000 [acre-feet] a year based on these assumptions’. That’s when we hired Jacobs Engineering to perform a feasibility study to take it to a little bit different level,” Brown said.

Brown and the UCRA were primarily concerned with the feasibility of catching, treating and utilizing the storm water within the City. In their study, Jacob’s Engineering addressed these questions and estimated a cost for building a pond where the storm water could accumulate before being sent to the water treatment plant.

Burt Weathersby, Senior Project Manager of Jacobs Engineering, presented the study’s findings to Council, explaining that the bulk of his study was based on a previously commissioned study conducted by the Texas Institute for Applied Environmental Research which provided data on previous storms. Weathersby said he used the data to approximate how much water comes off the 15-mile watershed during storms. 

It is recommended that they capture the 95th percentile of rain that comes down the watershed, Weathersby explained. This prevents the pond from being too large, but also ensures that it is capable of capturing the majority of the rainfall.

“We went and found a gage at Mathis Field to look at a 4-year period of record and we were able to determine that that 95th percentile rainfall was 1.67 inches…with the computer models that the TIAR had developed, and some of the other things we had looked at, we were able to determine that that 1.67 inches of rainfall yields about 1,000 acre-feet,” he said.

For this reason, the engineers determined they needed to build a pond that would hold at least 1,000 acre-feet of water, and settled on 1,800. The land anticipated to be used for the project is owned mostly owned by the City, a portion of it will have to be acquired. Should the City go through wit the project, the basin will be located at the downstream end of the watershed drained by Red Arroyo, a tributary channel of South Concho River.

Weathersby included in his presentation an optimistic estimation of the costs. With the vast majority of expense going to excavation, and a four-foot thick liner, a pumping system and relocating an existing water line included in the pricing, the total comes out to $20 million. The price is not fixed, however and could see a dramatic increase if not all goes according to plan.

“One of the things that makes this feasible is that we hope to use the land to the right of the where the pond will be built to stockpile and spread the material from the excavation of the pond,” Weathersby said. “If you can do that, it makes this $20 million cost estimate real. If you have to take that dirt and haul it off…that dirt cost—hauling—can go from $3 a cubic yard, which is in our current price estimate, to $10 a cubic yard, driving the cost of the whole pond up.”

From the City’s perspective, one of the biggest concerns with the feasibility of the project is an issue with ground water that could affect the liner. Councilman Rodney Fleming said he’d spoken with the City’s Water Department previously, and that there was worry that pressure from underlying groundwater may cause damage to the liner, which would require the use of a heavier, more expensive version.

“The cost includes a four-foot thick liner,” Weathersby said. “There are several ways to prevent the water from coming in…we looked at some of those forces that would press along that liner and possibly cause it to fail. It’s a shallow, unconfined aquifer and we think the forces that would be on that liner would be small enough to be held back by the four-foot liner."

“…we just don’t think this is going to be that big of deal,” Brown added. The liner issue and dirt options are areas that will need to be focused on in a later study.

On average the City of San Angelo uses 16,000 acre-feet of water per year. Although an estimated 10,000 acre-feet of water could potentially be caught by a pond at the meeting of the two Red Arroyo forks, the City would only be able to use half that for the municipal water supply. The issue stems from water rights that San Angelo has to various rivers and lakes in the area.

“According to the City’s water lawyers, you would be able to use 5,000 acre-feet of water—municipal supply,” Brown said. “The pond could continue to catch water and you could release that to downstream water rights. Currently, the City has to release out of Lake Nasworthy—Twin Buttes or Lake Nasworthy to the downstream. This pond would enable you to have another source to release to those, thus you can save water in your lakes.

The City has water rights all over the area and these rights can be transferred to other areas as needed. The City hopes to transfer a senior water right from the early 1900s that was appropriated to the City for Ben Ficklin Reservoir. Currently, the right is not being used, and would permit the City to pull 5,000 acre-feet a year and no more for municipal use.

In order to pull water from the Red Arroyo assuming the water right had been transferred, additional permitting would be required because the Red Arroyo is not classified as a river.  

“The Red Arroyo is considered a water course,” explained Attorney Jason Hill. “The water in it is state water regulated by the state of Texas, by the TCQ. Whatever water collects in the Red Arroyo is held in trust by the state. You’ve got to have a permit to be able to pull any of it out.”

Water attorneys have been working to seek options for the City in order to receive the permits and transfer the rights to make the project legally possible. Hill described the biggest issue at the moment as being a manpower issue, as individuals versed with the technicalities of the subject matter are needed to put in the work for a comprehensive application. They are currently working on a limited staff.

Should the permitting and construction become feasible, the Red Arroyo could provide the City with a third of its annual water supply.

Having heard the basics of the study findings, Councilwoman Charlotte Farmer indicated that she had three questions for Weathersby and Brown pertaining to implementation.

“What is your estimate of the quality of water and the cost to treat it to make it for human consumption?” Farmer asked.

“This is going to be a basin, so you would have to allow the pollutants to settle out. Whether you catch it in San Angelo or it goes down the river and settles down at Ivy, it’s all relative,” Brown said. “The treatment costs—there’s going to be a little bit more probably, because of the turbidity. The Red Arroyo is red, it has some fine clays, so there is going to be a cost to treat it that may be a little more. The settling effect will remove a lot of the pollutants. If it’s on the ground, it’s on the water. Generally speaking, the water quality as far as total dissolved solids is very good. There are some nutrients of course that run off—fertilizers and stuff like that from yards and just our every day use that are elevated, but not to an extent that it’s a concern.”

Brown explained that the stations set up in 2010 also take samples, which are then collected and sent of to laboratories. In the labs, the samples are tested for pollutants such as fertilizer and other things and the information is logged in a database that the UCRA maintains. According to Brown, the data shows that trends of certain pollutant types with heavier concentrations in certain areas. He hopes to use the information to mitigate pollution and to educate and raise awareness in the general public.

Following this response, Farmer asked how many gallons would be gained from the project so that she might assess the price of each gallon.

Brown explained that the project is available for a low, 2-3 percent interest water development board loan. Figuring roughly $100,000 per annum for operating and maintenance costs, Brown estimated that on a 20-year note at $20 million, an acre-foot of water would cost roughly $286, or $232 on a 30-year schedule. Should the project go 50 percent over budget and cost $30 million, an acre-foot would increase to $419.

“I will say that the previous studies done on water reclamation and water reuse from the wastewater treatment plant are anywhere from almost $900 down to a medium of about $700 from a study that was performed in 2006,” Brown said after presenting his numbers.

Farmer’s final question pertained to the time frame. Assuming permitting was put on the fast track, Brown estimates that within two years the pond could be up and ready for rainfall. The basin is designed to fill with storm water, which would settle for roughly a week and then be pumped out to the water treatment plant.

The plan relies on keeping the pool empty so that it will be ready for rainfall at all times. For this reason, it wouldn’t be possible to stock the pond with fish for recreational use for San Angelo citizens.

Before the project can move any further, more in-depth analysis will be required. Engineers and the City Water Department will look into the liner issue and hope to have an update at the next Council meeting.