Guidance for calculator to estimate chloride loading to groundwater from infiltration

This page provides guidance for using an Excel spreadsheet designed to estimate contributions of chloride to shallow groundwater from nine different sources. The calculator allows the user to adjust inputs for each source and produces an estimate, in pounds per acre or kilograms per hectare, from each source and cumulatively for all sources. The nine sources, described in greater detail below, include the following.

Infiltration through pervious surfaces
Infiltration through impervious surfaces
Leakage from water supply infrastructure, called piped inflow in the calculator
Leakage from the sanitary sewer system
Leakage from the storm sewer system
Infiltration from stormwater infiltration practices
Contributions from surface water
Seepage from constructed stormwater ponds
Infiltration from stormwater filtration practices

General information and limitations of the calculator

Before using the calculator, we strongly recommend reading the following.

When you open the calculator there are default values for all inputs. Each of these inputs must be evaluated for the conditions being considered.
The spreadsheet does not contain protected cells. Consequently, it is possible to delete formulas and default values.
An accompanying report called Impacts of stormwater infiltration on chloride in Minnesota groundwater provides detailed discussion and literature used to develop this calculator.

Structure of the spreadsheet

Rows 2-7: General inputs

Row 10: Pervious surfaces

Row 11: Impervious surfaces

Row 12: Piped inflow

This input includes leakage from water distribution systems. There is always some leakage from these systems since they are under pressure.

The default annual infiltration rate (I) is based on the following equation

\( I = P * F_{precipitation} * L \)

where

P is the annual precipitation (inches),
F_{precipitation} is the fraction of precipitation volume that is piped to the area being considered, and

L is the leakage rate from the piped infrastructure, as a fraction of total piped inflow.

The term F_{precipitation} is used because most literature expresses the volume of piped inflow as a percentage of precipitation. For example, studies suggest that a typical volume of piped inflow is about 50% the value of annual precipitation volume. Leakage represents an annual average value. For example, if 15% of the annual piped inflow is lost through leakage, the value for F is 0.15. The terms F_{precipitation} and L are thus unitless.

Users who have data for the volume of piped inflow should use that data. In this case, the formula in cell B12 would be rewritten as L times the annual volume of piped inflow divided by the area with piped inflow. For example, if the leakage rate is 0.15 the annual piped inflow is 1000 ft³, and the area with piped inflow is 100 ft², the annual infiltration rate (I) equals 0.15 * 1000 / 10 or 1.5 inches per year.