\section{Background}

Most U.S. uranium mines are classified as underground mines, surface mines \footnote{Either open cut or open pit}, or as in situ recovery facilities. The first step to establish a conventional uranium mine is to identify target orebodies. Then a shaft is generally sunk in the vicinity of the deposit and workings are excavated to remove the uranium ore \citep{nuclearregulatorycommission2020}. Blasted ore is brought to the surface and sent to a mill, where it is crushed or ground and processed into uranium concentrate. 
\begin{wrapfigure}{r}{0.37\textwidth}
	\centering
	\caption{In Situ Mining Process}
	\label{INSITU}
	\includegraphics{./Images/In_Situ_Diagram}
\end{wrapfigure}

In situ (in place) mines, leave the uranium ore body in the ground, and instead process low\footnote{Relative to the ore grades recovered with conventional mining methods} grade uranium deposits of sandstone that contain a ground water aquifer. A diagram of the basic in situ process is provided in \cref{INSITU}. A lixiviant\footnote{A lixiviant is any liquid chemical mixture designed to dissolve a ore concentrate \citep{wang2007}} designed to dissociate uranium from the rock is injected into the target formation. 

For Wyoming in situ mines the lixiviant is a mixture of native groundwater with typical additives such as carbon dioxide, oxygen, and sodium bicarbonate \citep{gregory2015,kehoe2023}, but international mines primarily use acidic lixiviants such as sulfuric acid \citep{worldnuclearassociation2024}. The acid or base dissociates the uranium from a sandstone roll front where a historic oxidation reaction deposited the ore \citep{wilson2015}. 

The lixiviant within a wellfield is pumped from the recovery wells to a plant that contains an ion exchange process. Groundwater from the uranium wellfields is passed through the ion exchange beads, which bind the uranium. Once the groundwater leaves, it is refortified with oxygen and carbon dioxide and reinjected into the mining aquifer within the wellfields. The pressure of the injection wells keep the solution within a closed loop in the aquifer. The resin beads, when fully loaded with the uranium, are transferred out of the ion exchange vessel and then stripped of the uranium in a process called elution. Clean resin beads are then transferred back to the ion exchange vessels for re-use. \citep{wichers2024a}

This process is repeated, cycling the groundwater between injection and recovery wells until uranium recovery rates becomes subeconomic, and the well grouping is retired. A single recovery facility serves a system of wells. As some wells are retired, others may be added further along the roll front, until all economically recoverable uranium is extracted, and the operation is ended.

%A common well system for in situ mines is referred to as a five spot. A five spot pattern exists when four injection wells are drilled in a rectangle, with a single recovery well in the center. These wells are shallow typically less than 100 ft deep. A piping system, often constructed with PVC pipes, brings the extracted water to processing facility and then back to the injection wells. This piping network is removed after operation are closed, and wells are capped.

In comparison to traditional mining methods in situ operations create minimal ground disturbance, do not produce tailings, and avoid expose of miners to elevated radon levels linked to lung concern \citep{nationalacademyofsciences1999}. The chemical injected into the groundwater, are commonly used in household without direct health risk, in Wyoming the most commons lixicant is sodium bicarbonate (baking soda).


 The \ac{SDWA} requires that in situ mine can only operate on \emph{exempt aquifers}. These are aquifers that the \ac{EPA} has identified as not being a suitable source of public drinking water. Either because the aquifer is already highly polluted, or because there are too few people in the area to make use of it. Once exempt, a aquifer can never be used as a public drinking water source.

 Other rules by the \ac{NRC} mandate that in situ mines restore the groundwater after operations are completed. Samples of groundwater are taken before the mine starts operation. These samples are tested for a array of dissolved solids. The rules require that thirteen different water constituent levels are returned to pre-mine levels. 

 In a typical restoration, multiple steps are taken to reduce post-mining increases in aquifer chemical constituents. The first stage of aquifer restoration is a groundwater sweep. The entire pore volume of groundwater within a wellfield is brought to the surface and injected into deeper layers through disposal well. The groundwater sweep process draws in native groundwater from outside the mining zone with lower dissolved solids refills this pore space \citep{saunders2016,yang2023}. Other projects manage the produced water using evaporation ponds, or water treatment followed by surface discharge \citep{wyomingdepartmentofenvironmentalquality2018}. If surface discharge is applied, the constituent concentration of the water must be tested before being applied to the ground \citep{wyomingdepartmentofenvironmentalquality2018}.

 Next, the water is run through reverse osmosis filtration and water treatment processes to reduce pollutants to allowable levels. Once these thresholds are reached, monitoring wells are used to track mineral content in a quarter mile buffer around the operation. If the decline rate of these factors is shown to be stable, the restoration is complete. \citep{internationalatomicenergyagency2016,saunders2016}. %Because each individual dissolved solids must be restored to pre-mine levels, some constituents are reduced below starting levels, as additional sweeps and filtering is used to remove the most difficult to extract pollutants. This was the case in the Smith Ranch Wyoming operation where radium levels in the groundwater were lower than baseline after restoration  \citep{ruedig2015}. 
