In coastal regions, MAR can be implemented to act as a barrier to seawater intrusion. Western states such as California see MAR as one important tool to enhance water security under changing climate.

Bhavna Arora portraitBhavna Arora is a research scientist in EESA’s Energy Geosciences Division who leads the Carbon Removal and Mineralization Program. Her work focuses on developing tools to provide a scientific basis for solving diverse issues in earth and environmental sciences, such as water resource management, nutrient cycling, as well as achieving net negative emissions.

She talks to us about her ongoing research on the use of MAR on agricultural lands and a recent publication she co-authored titled,  Agricultural Managed Aquifer Recharge (AgMAR) and Stratigraphic Heterogeneities on Nitrate Reduction in the Deep Subsurface.

Arora discussed the benefits of this approach and how it can be used to help address some of the water scarcity issues we face today.

Question: Why do you think it is important to conduct research related to MAR?

Answer: Water scarcity and insecurity are significant issues around the world. Climate change and population growth are stressing water resources more than ever, diminishing the amount of water available for human consumption and agriculture. Groundwater is an important player in the realm of freshwater. It is found underground, beneath our feet, and provides much of the world’s water supply for households and businesses. Given that groundwater is also an important water supply for agriculture, scientists have often used imagery to determine if the amount of water underground has changed over time. And, research has demonstrated that we’ve lost a lot of groundwater — especially in agricultural regions — and we need to think more about how to recharge this important resource. Computer modeling is a great way to explore related risks and opportunities.

Question: What is AgMAR?

Answer: First and foremost, AgMAR is a potential solution to some of the water scarcity challenges we face today. For some context, groundwater or aquifer levels vary over time. This variance is typically dependent on natural recharge where in a dry year, we may expect less surface water to infiltrate to recharge the aquifer. So, aquifers have a capacity similar to a surface water reservoir and good management of aquifers is crucial to dealing with water scarcity. What we can do is artificially recharge aquifers by directly connecting them to surface water sources during high flows. Agricultural Managed Recharge – or AgMAR, for short – is when we artificially recharge the aquifer on farms/croplands. AgMAR is a subtype of FloodMAR applications – the overarching term for all different kinds of MAR applications – that is specific to agricultural lands, whereas other FloodMAR strategies pertain to working landscapes or natural managed lands.

Question: Can you describe the key benefits of AgMAR systems?

Answer: It is a low-cost, low-energy water supply augmentation strategy that typically doesn’t require technological intervention as opposed to other techniques (e.g., desalination or building dams/reservoirs) used to obtain water for agriculture. In fact, by capturing surface water through AgMAR, we are helping manage floods and extremes during high flows. AgMAR also helps reduce surface water runoff that can pick up debris and nutrients on its way to the oceans, which may ultimately result in algal blooms (a quick increase in the amount of algae in the water source). AgMAR, therefore, presents a natural treatment to replenish over-exploited aquifers, which are typical in agricultural regions throughout the world.

Question: What are some of the important questions you and fellow researchers have about AgMAR?

Answer: Elevated levels of nitrate in drinking water have been linked to colorectal cancer, thyroid disease, and neural tube defects in babies. Therefore one of the main questions we are trying to address is when we apply on-farm water, what happens to fertilizer residues or the nitrate already present in soils and do we run the risk of polluting groundwater basins by leaching the nitrate? The nitrate that is applied on farmland as fertilizer is partly taken up by plants, and partly lost to the atmosphere, but most of it is retained in the soil. In order to address these concerns, we further explored what soil conditions and what frequency of application would be best for AgMAR with limited leaching of nitrates to groundwater. Clay soils, for example, hold on to nitrate due to their impermeable quality.

Question: Where did the research conducted for this paper occur?

Answer: The data is from an almond orchard in Modesto, California, which is located in the central valley of the state. The agricultural sector contributes significantly to the economy of the region (and the nation). We applied on-farm recharge and asked many of the questions I mentioned above. We found that finer-textured soils such as clays can prevent nitrate losses to groundwater. In contrast, coarse-textured soils such as sandy soils promote the leaching of nitrate. However, because soil textures in agricultural lands are not homogeneously made of sand or clay, we found that even small amounts of clay in coarse-textured soils can promote in situ nitrate retention. We also found that applying large amounts of water all at once under AgMAR is much more preferable rather than in small incremental amounts for preventing nitrate leaching to groundwater.