Brittany Kelley
AUSTIN — The Barton Springs-Edwards Aquifer Conservation District (BSEACD) Board of Directors heard a presentation regarding its requested hydrogeological and geochemical study at its meeting Thursday, Sept. 10.
According to BSEACD general manager Charlie Flatten, this study was commissioned a year ago and revolves around the Lower Trinity Aquifer — Sligo and Hosston. He noted that this is an update, though the final draft will be brought to the board in the coming months.
Stephanie Wong of LRE/Spheros explained that the project consisted of several tasks: data compilation and analysis, geochemical analysis, stratigraphy and structural evaluation, groundwater availability and recharge assessment and conclusions and recommendations.
“We didn’t have too much data within the district, so we also pulled in data from Hays Trinity [Groundwater Conservation District (GCD)]and then, kind of did a three-mile buffer around the two districts to consider any data in those areas, as well,” said Wong.
By pulling raw data from the Texas Water Development Board, drillers' logs and the groundwater database, the team was able to compare depths of completion to stratigraphy to narrow the district distribution to 340 wells. Nowadays, continued Wong, wells have to be reported when drilled and consist of descriptions of lithology and any test pumping done, which is what provides this data.
“I know a lot of the ones toward Caldwell County, some of them start to be oil and gas wells and then, sometimes, they’re just exploration wells.”
Most of the wells are concentrated in the north side of the Hays Trinity GCD, though some are still present in the southern portion.
“The reason that we see so many Lower Trinity wells in Hays County to the west is because they don’t have access to Edwards [Aquifer] water and because, especially on the northeastern boundary of Hays County, the Middle Trinity is exhausted and everybody’s having to do go down to the Lower Trinity to get water,” added Flatten.
Wong stated that this project was exciting because during their research, they discovered that there’s not any published rock geochemistry for areas, such as the Lower Trinity, within the district. So, they were able to sample from Spillar Well 1 and Reimers Ranch MW#3 and run labs on them, resulting in new geochemistry data for the Lower Trinity.
In the Reimers Ranch sample, they found basal conglomerate at the base, fine sandstone at the top and pebbles at the bottom in the core. The lab results showed low trace metals, said Wong. Additionally, the mineralogy indicated that the matrix of the aquifer is carbonate- and silica-dominated, which was expected.
This data can provide information on rock-water interactions and water chemistry, explained Wong.
On the topic of water chemistry, she explained that they were able to note where chemistry is different and the same.
“From this, we were able to see that there are trends, generally, of the wells that are more fresh and then, as you go north and east, you’re transitioning into an area with higher [total dissolved solids],” she said, sharing that they also analyzed radioisotope data. “We specifically pulled out the carbon 14 and tritium data because the analysis of that data [let’s us] get apparent ages … What we’re seeing in this map is that, generally, there’s younger water in the southern part of Hays-Trinity GCD and then, as you move northeast, the water is getting older.”
She stated that there is mixing happening between the two waters, which suggests that the Balcones Fault Line is having an impact on water chemistry in that area.
Wong also noted that the Lower Trinity has a large depth of rock and sentiment, making it “thick” the closer it gets to the fault line.
General groundwater direction is flowing from southwest to northeast, which is consistent with the trends of geochemistry they found, said Wong.
“The main takeaway, which I think we can all see, too, is that there is a big need for more data, especially within the district, to get a better handle on the Lower Trinity, in terms of geochemistry, stratigraphy [and other] properties, that’s local to here,” said Wong.
BSEACD will meet next at noon Friday, Oct. 16.
