An Open Source Spreadsheet for Calculation of Equivalent Freshwater Altitude in Brackish Water Mixing Zone of an Aquifer with Documentation on Appropriate Use

This software release documents an open source spreadsheet that was developed by the USGS in 2007 and modified herein for general use for calculation of equivalent freshwater altitude at wells where the water quality data indicate the well is tapping brackish areas of an aquifer system. One example of this includes coastal wells in the diffusion/mixing or transition zone of the aquifer near the freshwater/saltwater interface. This correction for salinity allows for the inclusion of water levels from brackish-zone wells in the construction of a potentiometric surface maps of areas with both fresh and brackish water present. The spreadsheet is not intended for computations of equivalent freshwater head at individual wells and does not include any adjustment in density owing to temperature. Typically, the reported water level in a well is the actual depth to water below a reference mark or the actual altitude of the water in the well. Equivalent freshwater altitudes are used when comparing water levels in wells in the same aquifer where some of the wells contain freshwater and some of the wells contain brackish water for inclusion in a potentiometric map (1,000 to 10,000 mg/L total dissolved solids; 400 to 8,000 mg/L chloride or density 1,005 to 1,010 kg/m3; essentially half the salinity of common seawater). Equivalent freshwater head is defined as the height of freshwater from a vertical datum that would create the equivalent pressure exerted by more dense water of a specific height from the same vertical datum. Thus, a common vertical datum should be established at the vertical midpoint of the freshwater part of the aquifer in the vicinity of all of the wells that will be contoured together and the altitude of that local vertical datum established. Water quality data (chloride concentration in milligrams per liter, specific conductance in micro-siemens per centimeter, or total dissolved solids in milligrams per liter) must be collected coincidentally with water levels to estimate density. All altitudes must be in a common vertical reference datum. This report summarizes the equation used and associated assumptions for the calculation of equivalent freshwater altitude within the spreadsheet and includes explanatory documentation as well as an example. The spreadsheet does not consider the temperature effect on density because it is small compared to the uncertainty of the altitude of the middle of the freshwater part of the aquifer. Additionally, the temperature in a confined aquifer remains fairly constant, thus all wells are assumed to be in water of similar temperature.