Irrigation Leader
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Removing Nitrates From Groundwater via Wood Chip Walls

Like many agricultural regions in the United States, New Zealand’s South Island deals with the contamination of its groundwater by nitrates. However, the situation is distinct in two respects. First, the alluvial gravel aquifers that underlie much of the intensively farmed land on New Zealand’s South Island are extremely permeable and are characterized by fast groundwater velocities. Second, the primary source of nitrogen in the groundwater is the waste produced by New Zealand’s millions of dairy cows.

In this interview, Lee Burbery, a scientist at the Institute of Environmental Science and Research (ESR) in Christchurch, speaks with Irrigation Leader about the technical, political, and economic challenges of combatting nitrate pollution and about the wood chip denitrification wall installations he is researching.

Irrigation Leader: Please tell us about your background and how you came to be in your current position.

Lee Burbery: After graduating with an environmental science degree from Lancaster University in the United Kingdom in 1996, I worked as an environmental consultant with the company ARCADIS Geraghty & Miller International, Inc. Most of my work involved conducting contaminated site investigations and toxicological

The mouth of the Rakaia River, a braided river that crosses New Zealand’s Canterbury Plains.

risk assessments. In 2000, I returned to Lancaster to complete a PhD.

My thesis concerned the limitations of single-well push-pull tests for measurement of in situ biodegradation rates in groundwater. At the time, monitored natural attenuation was developing as an approved groundwater remediation strategy and push-pull tests were evolving a conventional field- testing method.

In 2005, I moved to New Zealand to conduct postdoctoral research on the development of a recirculating tracer well test concept, which was a means by which nitrate reduction rates might be measured in situ in fast-flowing aquifers where single-well push-pull tests were impracticable. From 2008 to 2010, I did a stint as a hydrogeologist for the local regional council before returning to groundwater research. Seven years ago, I started working as a senior groundwater scientist at ESR, which is one of several government-owned science research entities referred to in New Zealand as Crown Research Institutes. The fate and transport of pathogens and nitrate in New Zealand groundwater systems are two of my major research themes. For many years I worked on projects aimed at evaluating the capacity of various New Zealand groundwater systems to assimilate nitrate pollution from intensive land use. More recently, this has bent toward the research and development of possible nitrate mitigation and groundwater nitrate remediation options. Since alluvial gravel aquifers typify New Zealand hydrogeology, they are the systems I am most interested in.

Irrigation Leader: Please tell us about the nitrate issue in New Zealand.

Lee Burbery: Agriculture has long been a mainstay of the New Zealand economy, and agriculture and nitrate pollution go hand in hand. Since the 1990s, however, agricultural land use in New Zealand has intensified significantly thanks to advances in irrigation, especially the use of groundwater pumped from high- yielding alluvial gravel aquifers. Driving this has been the massive expansion of the dairy industry. Accounting for 3.1 percent of gross domestic product, dairying now ranks as the fifth-largest industry in New Zealand. The national dairy herd is now 6.4 million cows— outnumbering the human population of 4.8 million. When you consider that a cow produces about 20 times more waste than a human, you can start to appreciate the scale of the agricultural waste problem on the small islands of New Zealand.

It is normal practice in New Zealand for cows to be raised outdoors on pasture rather than in barns, which makes dairy farming here more profitable. Although the use of nitrogenous fertilizers in New Zealand continues to rise, the dominant cause of nitrogen contamination is the urine patches from the millions of cows. Urine contains more nitrogen than soil can assimilate, and this nitrogen is leached into groundwater as nitrate and can present a health hazard when this water is consumed as drinking water.

I live in the Canterbury Plains area of the South Island, which is the largest flat-lying area in New Zealand and its most intensively farmed land. The plains are composed of alluvial gravel outwash deposited by braided rivers. The soils on the plains are young. You’re lucky if they’re more than 20 centimeters (8 inches) deep. Generally, they have a low water-holding capacity and are prone to leaching nitrate into the underlying gravel aquifers, and because alluvial gravel aquifers are inherently connected to the rivers that deposited them, that contaminated groundwater supplies the base flow to the rivers and spring-fed streams and lagoons in the lower region of the plains.

As a result of the recent intensification in agriculture, we are seeing a significant deterioration in our water quality, primarily due to nutrients, but also because of sediment and pathogenic bacteria. Water quality has become a salient issue in politics over the last 3 years, and there are actually politicians who have been voted in because of their promises to clean up the water and the environment. This is a challenging task, especially with regards to nitrate, because of groundwater lag effects. Some of the changes that are currently being observed are likely the legacy effects of postwar land-use intensification rather than results of the massive intensification of the dairy industry we’ve had in the last 20 years. Things are probably going to get worse before they get better.

Irrigation Leader: What are the health consequences of nitrate contamination?

Lee Burbery: New Zealand adheres to the World Health Organization advisory limit for nitrate in drinking water:
50 parts per million (ppm) of nitrate or 11.3 ppm of nitrate-N. That limit was set to protect bottle-fed babies against methemoglobinemia (blue baby syndrome), although I understand there has only ever been one reported fatal case of blue baby syndrome in New Zealand. Protecting public health is important, but it’s not the driver of concern at the moment. In New Zealand, rivers and lakes are naturally low in nutrients, and the aquatic organisms and plants are used to a low-nutrient environment. Therefore, even minor changes in the nitrate levels in surface waters—a rise to 2 ppm, for instance—can unleash an ecotoxic effect. We are already having issues with toxic algal blooms and both toxic and nuisance periphyton growths in rivers that are used for recreation and fishing. Rivers and water also have a huge cultural significance to the Māori population, especially in terms of mahinga kai, or food gathering. This population is seeing effects on its traditional areas for gathering crayfish, watercress, and fish.

The previous government came under some criticism from environmentally conscious folks for encouraging much more intensive agriculture. Its goal, as recently as 3 years ago, was to double productivity across the country while cleaning up the environment, which was not realistic. This was a significant reason that the present government was voted in—it promoted a more realistic approach, acknowledging that you can’t have it both ways without tremendous expense.

Irrigation Leader: How is your research addressing the problem?

Lee Burbery: My research is recognizing that the gravel aquifers are carbon limited. We do have a nitrate issue, and we don’t have extensive known areas where there is natural denitrification potential. Thus, I have been looking at the feasibility of passive, in situ groundwater nitrate remediation systems. We have had a lot of policy reforms in the last decade, including a national freshwater water framework that sets limits within water catchments on how much water can be taken out of a system that includes surface water and groundwater. Regional councils and populations are responsible for setting metering limits within catchments, although there are some upper limits that they cannot exceed, including drinking water quality levels and national limits on nitrate levels in surface waters, which vary depending on the type and location of the water body. Those limits are actually under review. The Australian and New Zealand Environment and Conservation Council established some guidelines that recognize ecotoxicity thresholds. In the case of nitrates, it has established levels as low as 1 ppm for 95 percent protection probability and to protect some of the more vulnerable species in our rivers. The previous government, which was formed by a conservative national party, bumped those limits up to 6.9 ppm. The current government envisions lowering levels back to 1 ppm. That is the driver behind all these remediation and mitigation practices and measures. While it may appear desirable, you have to question how feasible such a target is.

Irrigation Leader: Please tell us about your research into wood chip walls.

Lee Burbery: Nitrate is an oxidized form of nitrogen, and denitrification has long been recognized as the primary natural method for eliminating nitrate. It is a stepwise reduction process performed by consortia of bacteria, the end product of which is dinitrogen gas. The bacteria involved normally eat carbon or metabolize some substrate and respire oxygen, but when oxygen becomes limited, they switch to taking the oxygen off the nitrate, thus reducing it.

Wood chip walls promote the enhanced natural attenuation of nitrates. Wood chips are solid carbon and are porous, creating what we refer to as a denitrifying bioreactor. By providing organic carbon that allows the natural organisms of the subsurface environment to thrive, we create something like an anaerobic filter. We have aerobic water, so we have to create anaerobic conditions before any denitrification can occur.

There are situations in which you might put wood chips in a bed on a surface to filter surface water pipe drainage. That is becoming popular in the corn-growing regions of the United States. Another approach is to install a wood chip barrier in trenches deeper in the groundwater to produce a sort of constant denitrification process. This sort of wood chip wall is a porous, permeable, reactive barrier and can be constructed to ensure that water flows through and not around, it. It is still in the experimental stage, I believe. There is a gentleman named Casey Schmidt in Florida who put together an experimental version of such a barrier in 2012 and a nonexperimental version last year. His is the only active ongoing denitrification wall initiative that I am aware of other than my own.

While these other wood chip studies have been conducted in sandy aquifers of generally slow-flowing groundwater, we are trying to implement the technology in a challenging, heterogeneous gravel aquifer with high fluxes of water, nitrate, and oxygen. Because of our permeable gravel, we’ve had to make the barrier system highly permeable to prevent the water from simply flowing around it. The gravel channels within our aquifer have a permeability or hydraulic conductivity of about 10,000 meters a day, if not more.
That is as much as 100 times more permeable than sand. We’ve had to do a lot of work to come up with an optimal combination of correctly sized wood chips and gravel.

Irrigation Leader: What are the prospects for this denitrification wall process? Do you foresee its commercialization in the near future?

Lee Burbery: I do, particularly considering how the use of denitrification beds has become standard practice in the agricultural regions of the United States. The reason denitrification walls have been slower to evolve is their cost. Digging below the water table can be expensive compared to simply digging a trench in the soil and lining it with wood chips. Ultimately, the spread of the use of denitrification beds may depend on government policy. For example, if the new nitrate standard proposed by the New Zealand government were to become effective, farmers would need to dramatically cut the amount of nitrogen that is exported from their catchments. At that point, farmers will need to weigh the costs of farming with a denitrification wall against not farming at all.

Irrigation Leader: If installations like these became commercially and technically viable, would they be installed by individual farmers or by irrigation schemes and local governments?

Lee Burbery: Here in New Zealand, individual farmers would install them to address hotspots, but irrigation schemes and consortia might also use them to address the cumulative effects of leaching across a catchment. In general, these nitrate walls would need to be installed downstream of a particular pollution hotspot like a standoff pad. Perhaps if you had a large number of farms in a particular area, you could string a remediation system across the bottom of the catchment where all the water has to flow. It is likely that only larger organizations would have the resources and motivation to do that.

I should also say that denitrification is not always the best solution. For example, if we have aerobic water with some nitrate in it but nothing else, we have water that is not far from a geochemical standard of purity. New Zealand’s water is young and mostly free of heavy metals. But if we start targeting nitrate and make the water anoxic and chemically reductive, we start to mobilize inorganic chemicals like iron, manganese, and arsenic, which might be far more toxic. Similarly, we have to consider the greenhouse gas emissions created by rotting inorganic matter. In the process of denitrification, as it is on its way to being reduced to inert dinitrogen gas, nitrate-nitrogen is converted successively into nitrite, nitric oxide, and nitrous oxide gas, the last of which is a greenhouse gas 300 times more potent than carbon dioxide. If you underengineer something like this and it becomes carbon limiting, there is a risk that before it is converted to dinitrogen gas we could be pumping a load of nitrous oxide into the environment. We are studying and evaluating this sort of pollution-swapping phenomenon as part of our feasibility assessment. We are making significant progress in addressing the nitrate issue, but it is clearly a long-term initiative with many chapters yet to be written.

Lee Burbery is a senior groundwater scientist at the Institute of Environmental Science and Research in Christchurch, New Zealand. He can be contacted at lee.burbery@esr.cri.nz.