Jim Anderson

I recently received an email from an old friend I worked with on site evaluation and design questions over the years. He sent an article on new research methods being used to follow solute movement through soils. The article itself was specific to agricultural activities and was being looked at by the wine industry for the potential to help identify and rectify pesticide loss and environmental impacts. He asked if the approach and tools discussed have potential application in our decentralized wastewater industry.

The specific study he referred to was conducted by a group of Danish scientists trying to predict movement of pesticides and their degradation through soils to groundwater. They used visible/near-infrared spectroscopy on a variety of intact soil cores from agricultural fields to try to predict breakthrough curves for the pesticides and degradation through the columns. They showed good results in predicting breakthrough curves through the columns, which can then be used to predict how long it will take the various pesticides to move to groundwater from the fields.

The techniques used by the scientists are commonly used for medical applications such as assessing brain function and, as a quick screening tool, to identify issues such as bleeding in the brain. It does not replace MRI to identify these problems; but because the equipment used is more portable and can give faster results, it is very useful. This faster application is the reason for the Danish scientists’ study on soil columns.

NATURE’S FILTER

In a recent column, I discussed how separation distances were determined through research. This involves measuring how long it takes nutrients, bacteria and virus to move through soil columns. This is a time-consuming and labor-intensive process. The research I described went on for months. Any techniques we can use to speed information gathering in conjunction with solute models will help us understand better movement pathways through soils and have potential applicability to our industry.

The techniques used show great promise for better predicting soil treatment system impacts on the environment. But a lot of additional research and work is needed before these techniques will be of direct use to us in siting and design of treatment systems. The scientists recognized this for their specific application as well because they noted: “Soil is also the most important transport pathway for agrochemicals to groundwater. The soil’s ability to filter dissolved agrochemicals is dependent on the soil’s properties and the interaction between the dissolved solutes of those properties, and is influenced by how soils are used and managed.

“Soil structure is a very dynamic property since it is influenced by basic soil properties such as texture, organic matter, carbonates and metal oxides, climate, and land use and management practices. Depending on the soil structure, at close to saturation, water and dissolved chemicals can either be transported evenly through the soil or rapidly through specific pathways in the soil with various degrees of mass exchange between the soil matrix and the transporting pathways.”

This description captures the difficulty we have as well in terms of tracking our nutrients of interest — nitrogen and phosphorous — as well as bacteria and virus. Soils are not simply a collection of soil particles and soil pores (how they are arranged is hard enough to describe or predict); but also biological and chemical aspects that vary significantly. All these characteristics impact how virus and bacteria are captured and held and why transport of nutrients is not as direct as would be predicted.

POTENTIAL BENEFITS

At the same time I received the question about the study, I saw another article about a group of scientists who used a technique called nuclear magnetic resonance, or NMR, to investigate changes in moisture content of soil over time to better predict water movement. This method involves boring a hole with a well casing and using an NMR probe. Just a personal note here: We used an early version of this equipment to look at soil moisture content in the early 1970s.

They investigated this method because they recognized the variability due to the interactions in soil between the water and plants, microbes and minerals. They determined that if better predictions could be made, water managers and engineers could come up with better treatment systems and water management techniques.

Bottom line: I believe these investigations have potential applicability for our industry, but they are not currently techniques we can use on our day-to-day system investigations. With additional research, their results can help inform our decisions about design and operation of our systems. In the future, hopefully some of these techniques can be modified and developed for more widespread use beyond research applications.