Introduction

The salinity of groundwater and surface water across Louisiana remains an area of interest for agricultural commodities. Both horizontal (sea level rise and storm surges) and lateral (over-pumping and increased use of aquifers) saltwater intrusion have created a growing concern for tolerance of quintessential Louisiana crops. Though coastal communities see fluctuations in salinity from horizontal intrusion, central and northern Louisiana experience similar salinity problems from lateral aquifer salinity. Each of these cases has an impact on irrigation water used for our state’s agricultural commodities, which translates into economic decline. This article serves as a reference guide for salinity ranges and how to determine the salinity of your irrigation water.

Water Quality Terminology

• Salinity – The measure of all dissolved salts in water, which includes mostly chloride, sodium, sulfate, magnesium, calcium and potassium.
• Chloride – The measure of a single salt (contributor to salinity). Normally this is the most prevalent of salts in seawater.
• Total dissolved solids (TDS) – The measure of minerals, salts, metals, cations or anions dissolved in water. This analysis tests for inorganic salts, principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, sulfates and some small amounts of organic matter. In pure water, TDS is approximately equal to salinity.
• Parts per million (ppm) – The measure of concentration for total dissolved solids. For reference: 1 ppm is equivalent to 1 milligram per liter (mg/L).
• Parts per thousand (ppt) – The amount of salts in seawater is commonly measured in parts per thousand. For reference: 1 ppt is equivalent to 1,000 ppm.

Sampling Techniques for Water

There are many good service labs that can conduct salinity testing for irrigation water. The LSU AgCenter Soil Testing and Plant Analysis Lab has a routine water test package for irrigation and pond water. It is always best practice to have samples sent through certified labs for confirmation on any field test using handheld equipment. To decrease the volume of samples sent in for analysis, one could send confirmation samples on a specific schedule for your operation (e.g., monthly, quarterly) and test in the field with an electrical conductivity (EC) meter more frequently. An EC meter is an affordable tool to have handy for quick reference analysis of water. The EC meter measures electrical flow through the water using salt as a gradient. It is reported in units of microsiemens per centimeter (μS/cm) or decisiemens per meter (dS/m). There are many variables that can change the conversion of electrical conductivity to total dissolved solids (TDS), including nonlinear responses as EC increases and temperature. For simplicity, the following conversion is typically used for field-based calculations:

TDS (mg/L) = EC (dS/m) x 640 (EC from 0.1 to 5 dS/m)

TDS (mg/L) = EC (dS/m) x 800 (EC > 5 dS/m)

By using the simplistic calculation for TDS, the salinity (in parts per thousand) can then be calculated by the following equation:

Salinity (ppt) = TDS (mg/L) / 1,000

These formulas are simplistic field-level calculations for quick salinity estimates. The most accurate readings would consider temperature and ion concentration variables. For this caliber testing, samples should be sent off for laboratory analysis.

The following table shows calculations for μS/cm or dS/m readings that can be used for easy conversions using the above formulas. If salinity is an issue for your water source, printing the following page and laminating it for easy conversion of EC readings to salinity can be helpful.

Conversion Chart

See full PDF for more information.