Coalbed Methane Produced Water In The Western Us

BOX 2.2

Age of Groundwater

Hydrogeologists can test the validity of their conceptual models of groundwater flow systems, including predictions of where groundwater may recharge and discharge, by determining the approximate age of groundwater since it entered the subsurface as precipitation recharge. Fundamentally, an answer to the question "How old is the water?" can be determined geochemically (Bethke and Johnson, 2008). Knowing the age of groundwater, even within an order of magnitude, bears greatly on whether the water extracted from aquifers can be replenished by precipitation within human time frames, or can be considered "fossil" or ancient, nonrenewable water much like solid mineral resources such as coal and metals. Furthermore, directly determining the age of water sampled along subsurface flow paths also can be used to evaluate regional hydraulic properties used to calculate extractable water volumes. For decades, hydrogeologists have used various isotopes and other tracers to estimate groundwater age (Clark and Fritz, 1 997).

In groundwater "age dating," hydrogeologists assume simple plug flow (see figure below) and then correct age dates with respect to chemical processes (e.g., methane formation affecting carbon-1 4 dates) and hydrodynamic processes (e.g., diffusion of isotopes into fine-grained rocks or sediments surrounding the aquifer in question). Because of these complexities and others (see Bethke and Johnson, 2008), isotopic dating of groundwater usually can reliably be done at order-of-magnitude accuracy. In other words, analysis of a full suite of isotopes in groundwater can determine if the water is a few years old, tens of years old, hundreds of years old, thousands of years old, or millions of years old.

Isotopic age dating of water is based on elements that have multiple possible masses because of variable numbers of neutrons in thei nucleii. For example, the most common forms of isotopically stable carbon have atomic weights of 1 2 and 13 atomic mass units (written as 12C, 13C), in order of decreasing frequency. Isotopically stable hydrogen in water can have atomic weights of 1 or 2 atomic mass units (written as 1H, 2H). Oxygen can have atomic weights of 16, 1 7, and 1 8 atomic mass units (written as 16O, 17O, and 18O).

Some heavy isotopes of individual elements (those with highest atomic weights), particularly those of hydrogen and oxygen in water and carbon in organic and inorganic materials are sorted (fractionated) from their lighter isotopes as they move through the hydrological cycle and become involved in certain biochemical processes. For example, the fractionation of lighter from heavier hydrogen and oxygen in water causes water from precipitation

field and allowed deep fracture networks to propagate up through the rock units. These fractures enhanced the connectivity of otherwise isolated portions of the reservoir and now allow the CBM wells to effectively "mine" the connate water of the sedimentary formation. These connate waters were trapped at the time the original sediments were deposited tens of millions of years ago and are not being recharged.

The various datasets show that outcrops of the Fruitland Formation are not a significant mechanism for recharge of the coal aquifers. Rather, groundwater discharges to the surface at these outcrops in seeps that may have been active for millennia. Moreover, methane in the deep coal does not hydraulically connect to methane gas seeps at outcrops,

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