McGill, B. M. 2018. Climate change - groundwater interactions in a Midwestern US agricultural system and a peri-urban system in Botswana. Dissertation, Michigan State University, East Lansing, Michigan, USA.

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Theory predicts that groundwater-fed irrigation can contribute to greenhouse gas emissions and that climate change can alter groundwater quality and quantity. But few studies have tested or documented either of these concepts, which are of global importance to food and water security. For this dissertation, I have conducted two socio-ecological investigations: one in the Midwestern US where I found that groundwater use is augmenting anthropogenic greenhouse gas emissions; and the other in Ramotswa, Botswana where I found that climate change is exacerbating threats to groundwater quality.

The first socio-ecological system is Midwestern agriculture. Midwestern corn grain sales are > $40 billion a year, which is 60% of the US corn grain sales. Maintaining such substantial crop yields depends on nitrogen fertilizer, its associated lime requirements, and, in some places, irrigation. Both lime (CaCO3 or CaMg(CO3)2) and groundwater bicarbonate (HCO3-) alkalinity (via irrigation) are inorganic carbon © inputs to agricultural soils. The fate of this C and whether/how it contributes to climate change is not well understood. I conducted a field experiment at the Michigan State University Kellogg Biological Station (KBS) Long Term Ecological Research (LTER) site to investigate the fate of these inorganic C inputs in a no-till corn-soybean-wheat rotational cropping system. In Chapter 1, I show that irrigated plots had significantly less CO2 emissions from inorganic C reactions than rainfed plots, possibly due to increased carbonic acid weathering.

I used results from Chapter 1 along with measurements of several other components to assess the global warming impact of irrigation. These irrigation impacts are important to understand, because irrigated acreage in the Midwest is expected to increase in coming decades with less rainfall and more dry days in the summer. Though irrigation can help farms adapt to climate change, I found that its associated greenhouse gas emissions put it in a pernicious positive feedback loop with climate change. In Chapter 2, I demonstrate that no-till management flips from net C storage without irrigation to net C emissions with irrigation. Irrigation increased soil organic and inorganic C storage but not by as much as it increased fossil fuel use, soil nitrous oxide emissions, and nitrous oxide and carbon dioxide emissions from the
groundwater itself.

In order to minimize greenhouse gas emissions associated with irrigation or to encourage practices that induce greater C sequestration, we need to understand how producers using irrigation make decisions. In Chapter 3, I describe results from focus groups with irrigators and how their decision making process fits in a social theory context.

The second socio-ecological system is a peri-urban town in Botswana, an arid to semiarid country in southern Africa. The town is Ramotswa, and it is undergoing rapid urbanization and population growth. In Chapter 4, I demonstrate how climate change and social systems are compounding nitrate pollution of the Ramotswa aquifer. My results suggest that human waste via pit latrines are the source of contamination and that in situ denitrification could be used to remediate the groundwater.

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Human Surveys Resource Gradient

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