Spotlight on Best Management Practice at Antioch Cave on Onion Creek

No Drought
Lovelady well height: 522.41 ft-msl
Barton Springs: approximately 87 cfs 10-day average

No matter where you happen to live, the fact is you live on a watershed. That is, as scientist geographer John Wesley Powell described, “that area of land… within which all living things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community.” In other words, a watershed or drainage basin is a topographically unique area of land where all rainfall travels to the same destination. Central Texas is broken into several of these basins, many of which have recharge features that direct flow into the Edwards and other aquifers. You can use this tool from austintexas.gov to find out which watershed you live in, and how healthy it is.

Clean, natural recharge, in addition to healthy watersheds, is critical to replenishing all aquifers. Take a look at aquifer recharge in action at Antioch Cave, on the Onion Creek watershed about 1.3 miles west of Buda. Onion Creek is the main contributor of recharge to the Barton Springs segment of the Edwards Aquifer, supplying an estimated 45% of total recharge (BSEACD). While Antioch cave is the largest recharge feature on Onion creek, there are many features similar, but smaller that enable recharge. For example, check out Cripple Crawfish feature upstream of Antioch on Austin Water Quality Protection Land.

Antioch Cave, named after a nearby cemetery, is a particularly interesting recharge feature. It’s fitted with a sort of “self-filtration” system called a Best Management Practice, or  BMP. During the first portion of a rain event, water flushed into Onion Creek is more concentrated with sediments, bacteria, contaminants and other flood debris loosened while flowing over watershed terrain. To reduce the amount of these elements in water entering the aquifer, the BMP at Antioch, a boxlike, concrete structure uses two automated valves which open and close to divert contaminated water and allow desired recharge. Desired recharge is present in the latter portion of a rain event, after much of the contaminants and large debris, which could clog the cave and inhibit further recharge, have already washed out. So, the presence of this BMP structure reduces the amount of stormwater contaminants entering the aquifer through Antioch Cave. In addition, the BMP valve system also ensures continued flow along onion creek downstream of Antioch.

If you’re wondering how well the BMP does its job, consider this. BSEACD studies have shown that during one storm event in January of 2010, the amount of sediment, nitrogen and phosphorus prevented from entering the aquifer through Antioch was 16,907, 128, and 47 lbs, respectively (BSEACD p.3-12). To put that in perspective, that’s 8.5 tons of sediment--almost enough to outweigh this diesel truck! And while nitrogen and phosphorus occur naturally in soil and water, excess levels can be considered a contaminant of ground and surface waters and lead to damaging health effects for wildlife and people. Most sources of excess nitrogen and phosphorus come from human activity, usually traced to agricultural activities, human wastes or industrial pollution

Since the late 1990’s, the Best Management Practices at Antioch Cave and elsewhere along Onion Creek have played a critical role in improving the quality of water recharging the Edwards Aquifer and enhancing the amount of water recharging the aquifer along Onion Creek. For more information on the origin of the Antioch BMP, as well as photos and diagrams from construction, take a look at these appendices from KarstPortal.org.