To investigate whether poroelastic effects from recharge of groundwater bias the GPS motions, we analyzed 30 time series of wellhead levels obtained from the USGS around the Houston area and compared these to nearby GPS stations. Recharge of aquifers leads to elastic expansion and uplift that could be detected at nearby GPS stations. Here, we analyzed wellhead levels to estimate how much water diffused into the aquifer and increased the water level during the period of observed GPS uplift. If poroelastic expansion does affect GPS motions, then we would expect largest water increase during largest GPS vertical uplift rates. GPS uplift occurred over a period of ~5 weeks, however, with highest rates within the first 4 days.

We found that the comparison of most groundwater levels with vertical GPS displacement are inconsistent with surface motions expected from poroelastic effects. Almost all groundwater sites (87%) either show no deviation in behavior during Harvey or exhibit water level increases during largest GPS subsidence, which is followed almost immediately by a groundwater drop, when GPS uplift is observed. This surface motion is the opposite of that expected from poroelastic deformation. From the 30 groundwater levels, we only observed 4 that show an increase in water level during GPS uplift. However, we note that changes of water levels before and well after Harvey, which are similar in amplitude and duration (days-weeks) to water changes during Harvey, showed little or no response from the GPS stations. This would suggest that, from the current data, GPS stations are not sensitive to groundwater level changes during recharge over day-week time scales.

We estimated vertical displacement expected from poroelastic aquifer recharge using the wellhead data and assuming an elastic clay storativity that represents the Chicot and Evangeline aquifers (40). These estimates were then compared to the nearest GPS stations that were no further than 25 km away (fig. S6). To estimate vertical displacement of the aquifers (Δd) due to changes in wellhead level (Δh), we assumed a storativity coefficient (T) of 2 ×10−3 derived from (40), where Δd = Δh × T. From this more direct comparison, we found that the observed GPS motions were inconsistent with that expected from poroelastic motion from aquifer recharge (fig. S6). We found that, in almost all cases, the sign, amplitude, and functional form disagrees between the observed surface motions and that expected from poroelastic aquifer recharge. Instead, the aquifer levels can be interpreted to reflect direct changes in water across at the site and validate the filtered GPS time series.

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