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The Mystique, Mystery, Magic, or Marvel of Models

Case In Point - Predicting Ground Water Contamination Nitrate-Nitrogen and Our Health


Jim Bauder

James W. Bauder
Soil and Water Quality Specialist,
Montana State University

Background Paper

I think I began my interest in models about life below ground as a kid, digging holes in the "dirt" in a cornfield across the road from my parents' home in Tewksbury, MA. Maybe it was even before that, while picking rocks from my uncle's farm fields on a dairy in upstate New York. Anyway, wherever it started, today, four decades later, I find great fascination in trying to figure out just what it took to cause something to happen to our ground water or surface water. And, here in Montana, at the headwaters watersheds of the Yellowstone, the Madison, the Jefferson, the Gallatin, the Missouri, the Clark's Fork, there's always something going on that affects the water. The challenge for me today - learning the cause and effect relationships and then putting together "models", tools and templates so to speak, that explain

  1. how processes and practices on the land surface interact with climate, soil, geology, geography, plants, and social systems to affect ground and surface water quality; and

  2. how we can manage the watersheds in the present and future to sustain their desirable qualities and characteristics.
My interest - in the large scale land use practices that affect the ground and surface water quality.

I really got interested in modeling when I realized that most events in society affected physical environment which deal with land use are repeats - they have been done before and they will be done again. If only we could figure out the physical cause and effect relationship, then we could use the knowledge of these relationships to both prevent the occurrence of 'undesirables' in the future and modify our present activities and behaviors to produce predictable - and desirable - outcomes relative to surface and ground water quality.

Today my interest in water quality centers on much the same water quality parameters as it did in the early 1970s when I began work on an irrigation management project at Utah State University. My interest then - and now - was the behavior and characteristics of nitrate-nitrogen. Actually, the nitrogen cycle, but nitrate has a fascination of its own. Considering the significance of nitrogen - 79% percent of the earth's atmosphere, required by green plants, a structural component of proteins, amino acids, and many other life-essential compounds and structure - it has the potential to cause both acute and chronic toxicity when present as nitrate-nitrogen in drinking water. It also plays a significant role in estuarian eutrophication.

Nitrate-nitrogen is an intermediate product in the complete nitrogen cycle. The primary form of nitrogen taken up by plants, nitrate-nitrogen is anionic and hence readily mobile in soil systems. Nitrate-nitrogen is an intermediate product of composting, organic matter mineralization, atmospheric electrical storms; it is the intermediate product of the most abundantly applied plant nutrient on a global scale - nitrogen. Hence, being able to define and use the cause-and-effect relationships associated with nitrogen and nitrate-nitrogen in the environment to develop predictive models of nitrogen behavior has been a career of many scientists.

The "model" has become a popularly used tool to characterize and predict nitrogen and specifically nitrate-nitrogen behavior and occurrence. But, models serve more than just the single purpose of predictive tools. Some other outcomes or models:

  1. assessing impact, i.e., sensitivity analyses

  2. road maps for organizing information

  3. approaches to organizing thoughts and sequences

  4. emphasizing cause-and-effect relationships

  5. identifying gaps in information and understanding

  6. providing a central focus for discussion

  7. prioritizing the significance of relationships

  8. predicting future occurrences or consequences of actions

Along with models have come many problems, too. Case in point: no single model applies universally to all conditions or situations, i.e., certainly there will be some situation where the model provides inaccurate predictions or mis-representation.

And, there are is a wide variety of approaches that can be used in modeling. I myself have relied heavily on the statistically based, probablistic model in the past. If two events or parameters, i.e., a) independent variable and b) dependent variable, can be statistically related or correlated and scientifically argued or justified, based on solid knowledge, then the resulting probability has significance. What that means to me is that if water runs down hill and if streams rise when water enters them from upslope areas, there must be some correlation between the amount of water applied to upslope areas and the degree to which a stream rises in a given period of time after the water is applied upslope, i.e., physically based cause and effect.

Well.... That's my case with nitrate-nitrogen and models. For nearly three decades I have been trying to determine the cause and effect relationships that impact nitrate occurrence in groundwater. When I moved to Montana in 1980 I began studying fallowing - the practice of leaving land idle in preparation for the next crop. Sort of like composting in your garden. A few years ago, I started studying the occurrence and distribution of nitrate-nitrogen in Montana ground water. I then discovered a correlation (statistical at least) which existed between the extent of fallowing (composting as you would call it) and unexplainably high concentrations of nitrate-nitrogen in widely distributed geographic regions of Montana.

Back to measuring, monitoring, predicting and modeling nitrate-nitrogen occurrences in ground water.

It turns out that these areas that I discovered or reported for nitrate-nitrogen contamination of ground water were not the same as those predicted by some others using models based on farming practices and fertilizer use. Their models were based on the number of farms, the amount of fertilizer sold, and the amount of wheat production. My numbers were based on an actual sampling of nearly 2000 private wells across Montana.

Using the information I had available on high nitrate-nitrogen wells and MAPS, the Montana Agricultural Potentials System, a GIS based data overlay system, I assembled a statistically supported, probabilistic model, that allowed me to explain nearly 70% of the variability in my observations about distribution of nitrate-nitrogen in Montana ground water. The parameters that I found to be significantly correlated (which could be modeled) with nitrate-nitrogen concentration were:

  • the distribution of high permeability soils in a region
  • the percentage of a geographic area with slope greater than 8%
  • the percentage of a geographic area with slope less than 2%
  • the percentage of farming area 'not' irrigated
  • the percentage of a geographic area 'fallowed' (left idle)
  • the average 'water holding capacity' of the soil
  • the magnitude of difference between rainfall and evaporation

In the end, we eventually developed a model that allowed for the accounting of 50 to 70% of the variability in nitrate-nitrogen concentration in ground water using only two independent parameters, those being:

The percentage of a location in selected soil groups or types; and the percentage of a geographic area in land with slope greater than 2% and less than 8%.

Today, our investigations are moving ahead in a different direction. As you will see from a visit to a couple of the WEB sites posted here, statistics are beginning to surface (no pun intended) which suggest a potential correlation between non-Hodgkins lymphoma and Parkinson's disease and high nitrate-nitrogen concentrations in drinking water.

What is so important about nitrates in drinking water?

The following excerpts from a 1996 National Cancer Institute should get your interest sparked about modeling the occurrence of nitrate-nitrogen in ground water and surface water systems.

Date: Friday, September 6, 1996
NCI Press Office (301) 496-6641

Nitrate in Drinking Water Associated with Increased Risk for Non-Hodgkin Lymphoma (NHL)

"Contamination of drinking water with nitrate, a chemical in fertilizers, may be associated with an increased risk of non-Hodgkin lymphoma (NHL), particularly in agricultural areas, a National Cancer Institute study suggests. In a study published in the September issue of the journal Epidemiology, scientists from NCI, the University of Nebraska Medical Center in Omaha, and Johns Hopkins University in Baltimore assessed the average amount of nitrate consumed daily in tap water by Nebraska residents diagnosed with NHL, a cancer of the lymphatic system, and by a control group of persons without the disease who lived in the same area. Both groups used public water supplies....

The more nitrate they consumed in their water, the greater was their probability of developing NHL. Persons with NHL were twice as likely to be in the group that consumed the highest levels of nitrate as those without the cancer..... In Montana, high nitrate concentrations have repeatedly been detected in some locations in Fergus, Judith Basin, Roosevelt, Daniels, Sheridan, and Valley counties. However, it is uncertain whether the findings truly reflect the effect of nitrate, she added. An alternate possibility is that nitrate exposure is simply a surrogate or a Amarker variable@ that is correlated with another NHL risk factor that was not directly measured in the study.

Nitrate levels in ground and surface waters of agricultural regions have increased over the past 40 years as a result of increases in the use of nitrogen fertilizers and as a result of the long-term effects of intensive tillage and organic matter breakdown. Nitrate contamination occurs in geographic patterns related to the amount of nitrogen contributed by fertilizers, manure, and airborne sources such as automobile and industrial emissions, and to soil drainage characteristics. Areas with well-drained soils and high nitrogen inputs have the highest nitrate levels in the water supply. In particular, large areas of the Midwestern corn belt states have nitrate levels above natural levels. The U.S. Environmental Protection Agency's regulatory limit for nitrate is 10 mg per liter of drinking water."

This document is available through the NCI's CancerNet services on the Web (http://cancernet.nci.nih.gov) and through Cancer Fax (dial 301-402-5874 from the handset on your fax machine).


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