Environmental Stewardship takes action to protect the Colorado River by petitioning Lost Pines GCD to adopt Desired Future Conditions on the Colorado Alluvium Aquifer
September 24, 2022
Environmental Stewardship took a major step to protect the Colorado River from the impacts of groundwater pumping by filing a petition with Lost Pines Groundwater Conservation District (Lost Pines GCD) to adopt desired future conditions (DFC) for the Colorado Alluvium Aquifer. Groundwater from the aquifers in Bastrop County, Texas, flow into the Colorado Alluvial Aquifer where water is exchanged between the river and the alluvium. Under current and historic conditions, the net flow of water is from the aquifers into the river. This flow supports sound ecological conditions in the river and are especially important during drought conditions.
The petition requesting that Desired Future Conditions (DFCs) be adopted and implemented for the Colorado Alluvial Aquifer in Bastrop Count, Texas, was filed with the Lost Pines GCD Board and staff on September 20, 2022.
Environmental Stewardship is requesting that a desired future condition be established for the Colorado Alluvial Aquifer with the following objective:
Objective: To maintain the saturated depth of the Colorado Alluvial Aquifer at a minimum of [to be determined] feet above the most recent drought of record “DOR” low-flow water level in the Colorado River to maintain a relationship between the river and the alluvium that is adequate to support a sound ecological environment during a repeat of the most recent DOR in the Bastrop reach of the river and protect surface water rights throughout the basin.
The petition and appendices include a great deal of new science work by George Rice, our resource consultant, in support of the petition and legal guidance by Eric Allmon. This is the first step in getting DFCs adopted and installed to protect the Colorado River and property rights (surface water rights and domestic wells) from the impacts of groundwater pumping in the District and Groundwater Management Area 12 (GMA-12).
In the Lost Pines Board meeting on September 21, 2022, the Board agreed to consider Environmental Stewardship’s proposed amendment to the District’s Management Plan concerning the protection of the river and surface water rights and property rights as follow:
Goal: Address surface water and groundwater interactions in the Colorado Alluvial Aquifer to enable management of the impact of groundwater pumping on the Colorado River and its tributaries, to maintain a relationship between the river and the alluvium that is adequate to support a sound ecological environment during a repeat of the most recent DOR in the Bastrop reach of the river and protect surface water rights throughout the basin and protect property rights.
Objective: To better understand how, and to what extent, local aquifers contribute to streamflows under wet, dry, and drought conditions. Determine what level of contribution sustains a sound ecological environment and protect property rights through extraordinary drought conditions.
The District will be holding a public hearing on amendments to the Management Plan on October 19, 2022 (public notice is in process). This is an essential first step in the process of adopting DFCs on the Colorado Alluvial Aquifer as requested in Environmental Stewardship’s petition.
A MAP AND A BIT OF SCIENCE TO ORIENT TO THE SUBJECT OF THE PETITION
Figure 2 (numbering follows the petition) shows the extent of the Colorado River Alluvium in south-central Texas. Jeoffrey P. Saunders (1996) described the geology and hydrology of the river and alluvium, and Environmental Stewardship agrees with his characterization (see Appendix 5 in the petition). Studies have consistently found that as duration of a drought increased, bank storage is depleted and river baseflow is dependent upon aquifer discharge (outflow) to surface water.
Figure 2. Extent of the Colorado River Alluvium, south-central Texas (after Barnes, 1974)(Appendix 1, Figure 2 of Petition)
The Colorado River Alluvium (also known as the Colorado Alluvial Aquifer) exchanges water with the river. The river alluvium also exchanges water with other alluvium that is not in direct contact with the river, and with the underlying shallow flow layers. The alluvium also receives recharge from precipitation. The flow relationships simulated by the GAM are illustrated in Figure 6 below (Figure 1 of Rice Report, Appendix 8).
Figure 6. Schematic of flow relationships simulated by the GAM. (Figure 1 in Rice Report, Appendix 8)
Figure 7 shows GAM predictions of net flows between the river alluvium and the Colorado River, the other alluvium, and layer 2. Pumping from Lost Pines GCD and the contribution from recharge is also shown. As shown in figure 7, the river alluvium receives most of its water from the shallow layers (represented by layer 2 in the GAM) and from the other alluvium that surrounds it. The GAM simulation can be summarized as a series of events that begins with pumping and ends with reduced flow to the Colorado River. These events are outlined below.
- Pumping reduces water levels in the underlying aquifers
- The reduced water levels induce flow from the shallow layers (Layer 2) into the underlying aquifers
- Because more water is flowing from layer 2 into underlying aquifers, less water from layer 2 flows into the river alluvium
- Less water in the river alluvium results in less flow from the alluvium to the Colorado River
Figure 7. GAM predictions of flows to and from the Colorado River using S-19 pumping file (Figure 2 in The Rice Reports, Appendix 8).
Rice provides several additional graphs to illustrate the details regarding the magnitude and direction of these flows (See: Rice Report, Appendix 8).