Managed aquifer recharge with marginal water for irrigation and contaminant attenuation

Summary

Water resource scarcity, food security, and environmental pollution are three major issues, that threaten the sustainability of the natural environment and the lives and livelihoods of people around the globe, especially the marginalized and those who live in developing regions. Despite increasing efforts by scientists and policymakers to resolve these issues, there is still no end in sight, partly because of the lack of conviction, and partly due to inability. With currently available technology, it is often necessary to make sacrifices regarding one of the issues in order to effect an improvement in another. For example, attempts to tackle food shortages by increasing agricultural production would necessitate the use of more water resources for irrigation, and agricultural chemicals for soil fertilization, which in turn may exacerbate water scarcity and environmental pollution, resulting in no net benefit to sustainability. It is therefore imperative that methods to resolve such issues that have minimal trade-offs are developed.

A new method of agricultural irrigation that has the potential to alleviate the above three issues is currently being tested at an experimental agricultural site in the Netherlands. This new method involves a newly developed subsurface irrigation and drainage system, irrigated with treated wastewater. To justify the use and development of this new method, Chapter 1 provides some historical context and a brief technical description of the method, along with an introductory discussion on its risks, benefits, shortcomings, and advantages in terms of environmental sustainability and crop contamination. Wastewater, including treated wastewater as no treatment technique can fully remove all impurities from wastewater without consuming large amounts of energy, contains residual biological and chemical substances. Some of these substances are known as contaminants of emerging concern (CECs), and are toxic or ecotoxic substances that are poorly understood in science due to reasons such as novelty or rarity. Hence, irrigating treated wastewater directly onto crops would contaminate the crops and the food supply.

Treated wastewater irrigation with minimal risks of crop exposure may be accomplished by subsurface irrigation through pipes buried in the phreatic zone, which is situated some distance beneath the root zone. Part of the agricultural water demand may thus be fulfilled by applying treated wastewater to the soil through these pipes, upon which they raise the water table and increase the capillary flux towards the root zone, thereby irrigating the crops without directly exposing them to the CECs in the treated wastewater. CECs transported upwards by the capillary flux would be at least partly biodegraded by microorganisms present in the soil, in addition to being diluted by the groundwater, and being adsorbed to the soil matrix. Therefore, the water that reaches the root zone through capillary rise should contain a much lower concentration of CECs than the irrigation water. On days with large precipitation fluxes, or outside of the annual crop season, the same pipes can be used to drain the soil, and remove a portion of any CECs that may remain. Nevertheless, there remains a risk that crops will be exposed to CECs, or that the CECs may be transported in the subsurface to deeper groundwater aquifers, which is a source of freshwater, or surface water at the end of the phreatic aquifer, whereupon the CECs would pollute the aboveground environment. Subsurface irrigation and drainage with treated wastewater falls under an umbrella of environmental technologies known as managed aquifer recharge (MAR). An objective of this thesis is thus to evaluate the risks that irrigated CECs from such an MAR system would contaminate crops or the wider environment in significant quantities.

In Chapter 2, a model of subsurface irrigation and drainage with treated wastewater is constructed and validated against data from an experimental site in the Netherlands. A sensitivity analysis of the model is conducted in Chapter 3, where the main conclusion is that the fate of irrigated CECs and the risks of crop and environmental contamination are primarily dependent on the biogeochemical behavior of the CECs and the hydrogeological conditions of the subsurface. The only mechanisms by which the irrigated CECs may be removed from the subsurface without contaminating crops or the wider environment is for them to either be drained away or to be biodegraded in the soil. Accordingly, another objective of this thesis is thus to study the recovery efficiency of MAR systems in general, from a theoretical perspective, through analytical and numerical modelling: Chapter 4 and 5 analyzes the recovery efficiency in spatially homogeneous and heterogeneous aquifers respectively, while Chapter 6 studies the in-situ biodegradation of contaminants in the subsurface.

Altogether, the results of the work presented in this thesis suggest that treated wastewater irrigation with the introduced subsurface irrigation and drainage system is safe for crops and for the wider environment at the experimental site and in regions with similar hydrogeological and climactic conditions, as long as the treated wastewater does not contain excessive concentrations of highly mobile and persistent CECs. Although the drainage of CECs through the buried pipes contributes little to CEC removal from the soil at the experimental site, this may change under different hydrogeological conditions, such as if the regional groundwater flow velocity is slower. Extreme outcomes related to CEC spreading in the environment, as might be predicted by simpler models of CEC transport and transformation, are less likely to be observed when models accounting for more realistic conditions are used, suggesting that the tail risks of crop and environmental contamination would be overestimated by simpler models. The results also suggest that in many cases, but not all, crop and environmental contamination outcomes might improve on a relative basis as the irrigation volume and operational history of the system increases due to the nature of dispersion under radial advection and due to microbial adaptation, which is encouraging for wider adaptation. In conclusion, the introduced subsurface irrigation and drainage system is effective at safely fulfilling its purposes, as long as care is taken to avoid irrigating the subsurface with water that contains contaminants that are highly persistent in the soil.