HYDROLOGY AND CHANGES IN SHALLOW GROUNDWATER GEOCHEMISTRY AS A RESULT OF CO2 INJECTION AT THE ZERT SITE, BOZEMAN, MT

The Zero Emissions Research and Technology (ZERT) group, led by Lee Spangler (MSU, Bozeman, MT) is a multidisciplinary project that is developing tools for near-surface monitoring and detection of CO₂ associated with deep geologic carbon sequestration. The ZERT Site resides in an agricultural field at Montana State University in Bozeman, MT. To simulate a leak from a deep sequestration site, CO₂ is injected into shallow groundwater through a horizontal perforated pipe situated 2-2.3 m below the ground surface. The water table is 1-1.5 m below the ground surface during the summer months.

In the summers of 2008, 2009, and 2010 the USGS investigated changes in groundwater geochemistry at the ZERT site during CO₂ injections. Water samples were collected from ten monitoring wells and analyzed immediately for pH, alkalinity, and electrical conductance. Aliquots were filtered, preserved, and analyzed in the laboratory for major, minor, and trace cations and anions. A constant discharge pumping test and a natural gradient tracer test were conducted in July 2010 in order to measure hydraulic parameters.

Rapid and systematic changes were observed in the chemical composition of groundwater in response to CO₂ injection. During the 2009 injection when 200 kg/d of CO₂ was injected, the groundwater pH dropped to 5.8 from a background level of 6.6. Water alkalinity and electrical conductance increased (from 350 to 900 mg/L as HCO₃, and from 400 to 1200 μS/cm, respectively). Results clearly show that the addition of CO₂ created more acidic groundwater which facilitated the dissolution of soluble phases, most notably calcite. The increase in calcium, magnesium, and trace metals in water samples supports this conclusion. Hydraulic tests conducted in 2010 indicate lateral groundwater flow is rapid (pore water velocity ~2 m/d).

The release of CO₂ into a shallow groundwater aquifer at the ZERT Site has invariably changed water chemistry including decreasing pH, and increasing the electrical conductance and alkalinity. Determining the aquifer parameters and changes in water chemistry are important for understanding the impact of a CO₂ leak on groundwater.