A comparison of anion concentration in surficial groundwater sampled from two types of water quality monitoring wells

Groundwater sampling for monitoring the presence and concentration of contaminants can be done using either depth integrated monitoring wells (MW) or depth specific multi-level sampling (MLS) wells. Depth specific multi-level sampling wells (MLS) are cost-effective, easy to install, and provide very detailed information about the vertical gradient in contaminant concentration. In contrast the MW sampling provides information on the presence of contaminant over large representative area. This study was conducted in two 33 ha blocks of a commercial citrus grove (Valencia orange trees on rough lemon rootstock) in a well drained Ashtabula fine sand (hyperthermic, uncoated, Typic Quartzipsamments). The depth to surficial groundwater at monitoring locations varied from 1.4 to 5.6 m, and the lateral groundwater flow rate was approximately 0.08 m d⁻¹. Anions were measured in groundwater sampled at 3-week intervals from four pair of MW and MLS in each of the two blocks. Since the screened portion of the MW in this study was placed in the top 150 cm of the surficial aquifer, the sampling parts of the MLS within this depth (2nd and 3rd ports) were considered for comparison. The results showed that the concentration of NO₃ ⁻-N, SO₄ ²⁻ and Cl⁻ in the MW samples were similar to the mean of the 2nd and 3rd port MLS sample concentrations over a one year period of sampling. Therefore, MLS sampling provides a technique to assess the groundwater quality very similar to that which can be obtainable by MW technique. In addition, the MLS provides useful information on the vertical gradient of solute concentrations thus allows evaluation of the short-term impacts of land management changes on solute concentrations in the very top layer of surficial aquifer in cost-effective manner.

Keywords:

• Ground water
• Nitrate nitrogen
• Sulfates
• Chloride
• Multi-level sampler
• Monitor well

Acknowledgments

This study was made possible, in part, by a grant from the Florida, Department of Environmental Protection (FL DEP), USA. The authors gratefully acknowledge the assistance of Dr.Wendy D.Graham for providing valuable suggestions which resulted in significant improvement of the paper.