Effect of tropical storms Irene and Lee on groundwater levels in well Qb35-08

Groundwater levels recorded in Qb35-08, a 14-foot deep monitoring well located approximately 5 miles west of Laurel, DE show a remarkable response to tropical storms Irene and Lee, which occurred in August and September, 2011, respectively. Groundwater levels and temperatures in Qb35-08 were collected with an automated pressure-temperature datalogger system. At the same time, rainfall and soil moisture data were recorded by the DEOS Laurel Airport station located approximately 5 miles from the well. In the following illustrations daily mean groundwater levels and groundwater temperatures, total daily rainfall, and daily maximum volumetric soil water content are plotted to show how groundwater, rainfall, and soil moisture are related.

Note the two big storms were 6.6 (Irene, August 27-28) and 3.7 (Lee, September 8) inches. In response, groundwater level (gwl) rose more than 9-1/2 feet, with the largest rise occurring after the second storm. Though a 9-1/2- foot rise in groundwater level within a few weeks is remarkable, long term the gwl are within the range of previously observed gwl. The shallowest groundwater levels observed in this well, about 3 feet below land surface, are very common in this part of Delaware and a primary reason why there are very few homes with basements in the area. Daily mean groundwater temperatures do not show a noticeable affect from the storm events. The timing and magnitude of groundwater level rise is related to the rainfall amount and the antecedent maximum daily volumetric water content (VWC) or soil moisture conditions. Prior to the first storm VWC was less than 0.1 indicating a significant soil moisture deficit. Increase of VWC to values above 20 percent (0.2) and onset of gwl rise appear to be strongly correlated with a delay of one day or less. Rates of increase in gwl slow within two days and reverse within 3-4 days after the VWC drops below 20 percent. The rapid rates of rise in VWC and gwl reflect rapid infiltration and groundwater recharge and are consistent with the sandy soil at the DEOS station and sandy aquifer material at the well site. Rapid recharge is one reason why the shallow aquifer is especially vulnerable to pollution from contaminants released at or just beneath land surface. These data provide an example of why it is important to monitor groundwater conditions. Well Qb35-08 is operated by the Delaware Department of Agriculture as part of their Pesticide Compliance Monitoring Network (PCMN, http://dda.delaware.gov/pesticides/gwater.shtml). Construction, maintenance, and instrumentation are done in cooperation with the Delaware Geological Survey. Additional groundwater and well data can be obtained from this web site or from the PCMN web site.