Abstract
The hydraulic head is the most important parameter for the study of groundwater. However, a head measured from observation wells containing groundwater of variable density should be corrected to a reference density (e.g., a freshwater head). Some previous case studies have used unknown density hydraulic heads for calibrating flow models. Errors arising from the use of observed hydraulic head data of unknown density are, therefore, likely one of the most overlooked issues in flow simulations of seawater intrusion. Here, we present a case study that uses the freshwater head, instead of the observed hydraulic head, to analyze the flow paths of saline groundwater in the coastal region of the Pingtung Plain, Taiwan. Out of a total of 134 observation wells within the Pingtung Plain, 19 wells have been determined to be saline, with Electric Conductivity (EC) values higher than 1500 μS/cm during 2012. The misuse of observed hydraulic heads causes misinterpretation of the flow direction of saline groundwater. For such saline aquifers, the determination of a freshwater head requires density information obtained from an observation well. Instead of the purging and sampling method, we recommend EC logging using a month interval. Our research indicates that EC values within an observation well within saline aquifers vary not only vertically but also by season.
Highlights
Seawater intrusion is one of the most concerning issues for groundwater resources management within coastal aquifers
Using hydraulic head observations to infer groundwater flow directions is a basic procedure in seawater intrusion studies [1,2,3]
Errors arising from the use of observed hydraulic head data of unknown density are, likely one of the most overlooked issues in flow simulations of seawater intrusion [14,15,16]
Summary
Seawater intrusion is one of the most concerning issues for groundwater resources management within coastal aquifers. Using hydraulic head observations to infer groundwater flow directions is a basic procedure in seawater intrusion studies [1,2,3]. Organizations such as the USGS, The Geological Survey of Denmark and Greenland (GEUS), and the Taiwan Water Resources Agency collect long-term hydraulic head (water-level) data and maintain observation well nets (Table 1). A hydraulic head is obtained by measuring water level in an observation well where level refers to mean sea level Numerical models such as SEAWAT are used to simulate variable-density transient groundwater flow in porous media, using freshwater as a reference fluid [4]. We provide recommendations for water level data collection from a saline groundwater area
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