Soil Pollution From Recycling Centers: Case Study Analysis from the Campus of Michigan State University

Discussion

The trends in average elemental concentrations give insight on the interactions occurring between the MSU recycling center outputs and surrounding soil intakes. The initial question of which tradeoffs accompany a recycling center was addressed by analyzing soil taken from Baker Woodlot and the recycling center. Our original hypotheses stated that levels of pH, nitrate, nitrite, ammonia, aluminum, phosphorus, and ferric iron would differ substantially between soil from a natural area and soil exposed to anthropogenic change. Increased traffic attracted to the recycling center also would influence test variable concentrations, showing a correlation with distance from roads surrounding the MSU recycling center. In accompaniment, if the recycling center strongly influenced the variables in the soil from the closest edge of Baker Woodlot, the mean concentrations would be similar to those of the soil from the recycling center.

Observed pH levels displayed the adverse effect of the nearby recycling center. Soil from the recycling center dropboxes to the outer edge of Baker Woodlot was less acidic and closer to neutral levels than the controls. Only pH supported our third hypothesis that the recycling center influenced soil composition at the edge of Baker Woodlot since pH levels were consistent between the recycling center and 120 meter data.

In contrast to higher pH values, nitrate levels were lowest in the soil closest to the recycling center. This is likely due to the fact that forest ecosystems (particularly maple trees) are much better at increasing nitrate levels (Mitchell, 2011). There was very little vegetation in the vicinity of the dropboxes and a sparse line of coniferous trees around the perimeter, both of which are not as efficient at generating nitrate. The reverse of this effect could also be an important factor; nitrates are the limiting factor in plant growth. The low concentrations of nitrate could be the reason for the patchy vegetation observed close to the dropboxes. There was a strong positive correlation of increasing nitrates with distance from the recycling center. Since concentrations were different between recycling center samples and natural protected areas, there is support for an influence of the recycling center on the surrounding soil composition. However, the positive correlation with distance from the road is more likely due to forest composition and seasonality than inputs from the recycling center.

The nitrate results are also related to the ammonia and nitrite concentrations due to involvement in the nitrogen cycle. Only samples near the recycling center registered for ammonia and nitrite (one control tested for ammonia alone). This contributes to our second hypothesis, which stated that increased traffic emissions around the recycling center influenced the nitrogen concentrations. Nitrogen oxides are a common emission from motor vehicles (Twigg, 2006). New technologies in controlling these nitrous oxides involve using ammonia and oxygen to break down nitrous oxides into nitrogen gas and water in a process called Ammonia SCR (selective catalytic reduction) (Twigg, 2006). However, excess ammonia could be emitted into the atmosphere (LePree, 2010). Without vegetation to efficiently generate the nitrates, nitrogen oxides and ammonia emissions slowly move through the nitrogen cycle, which could explain why ammonia and nitrites were found together in dense concentrations.

Another important variable to our study is aluminum concentration. Aluminum showed a strong negative correlation with distance from the recycling center. The trend followed our predictions and supported our first hypothesis that outputs from material gathering and processing would influence aluminum concentrations in the surrounding soil. It is most likely due to aluminum leaching from the material collection boxes. Aluminum is the second most widely used metal in the world and makes up a large portion of recycling center materials (Olivieri, 2006).

Similarly, phosphorus levels showed a strong negative correlation with distance from the dropboxes. This shows strong support for our first and second hypotheses that the recycling center and increased traffic emissions have an influence on soil composition in the area.

In contrast, averages of ferric iron concentration showed no correlation with distance from the dropboxes; we would fail to reject the null hypothesis that the recycling center has no influence over ferric iron composition.

When the analyses from each element are combined, it is apparent that some evidence has been collected in support of the MSU recycling center altering soil content in the surrounding ecosystem. Almost all elements were different between the recycling center and the areas protected from anthropogenic effects, establishing overall evidence to reject our null hypothesis that soil composition is not influenced by outputs related directly to the recycling center. Since the MSU recycling center has only been in operation for two years, and the anthropogenic effects on soil nutrient content within Baker woodlot are already evident, even greater trends may become more apparent in the future.

A similar study compared soil composition between various types of land use, including natural areas and those exposed to anthropogenic change. Similar to our results, the most acidic areas they found were natural forests and reforested areas (Islam and Weil, 1999). They suggested high aluminum concentration in the forest areas contributed to the acidity (Islam and Weil, 1999). This supports only part of our data; our recycling center samples were highest in both aluminum and pH. The difference may be because the recycling center dropboxes were recently built and the aluminum may not have had enough time to influence the pH of the soil. Also, Islam and Weil had other factors affecting their pH values, such as monsoon seasons and resulting changes in mineral concentrations (1999).

Similarly to Islam and Weil’s results, some of our data may be skewed due to the time of the year at which our samples were collected. Changes in soil composition correlate with environmental changes through the season, such as plant decay, cooler temperatures, and lack of photosynthesis. Another methodological problem inherent in this study is the fact that a lot of the tests we ran only had broad scales of measurements, preventing finer records of trace amounts. Ammonia and nitrite concentrations were difficult to analyze as very few samples were concentrated enough to register on the scales provided for the tests. In future studies, we would obtain a way to measure more precisely and gather significantly more replicates to increase the accuracy of our results. In addition, inconsistencies in soil depth between experimenters may potentially be a source of error, so, in future studies we would test sample fractions of <200micrometers, as Papastergios et al. (2010) did. Amount of traffic is also a dynamic variable that should be accounted for in our results. It is important to emphasize that, because a number of variables were not being controlled for, no definitive statements about causation are inferable from our results. Regardless, any correlations we found could provide an interesting foundation upon which to conduct further research.Continued on Next Page »