Title: Carbon dioxide fixation by calcite and diffusive degassing in the southern region of the Berlín geothermal system, El Salvador

Abstract

Two processes of carbon dioxide removal from geothermal systems have been studied as part of the
CO2 budget in the Berlin geothermal system and their implications for the evaluation of the CO2
emissions from a 109 MWe geothermal power plant. CO2 flux through soil was measured as part of
natural emissions at 150 sampling points within a regular grid of 4 x 2.5 km using the accumulation
chamber method. Results were analyzed by a graphical statistical method. Soil CO2 flux values over
background values were not observed and characterization of the CO2 source feeding the soil CO2
diffuse degassing was not possible due to the amount of volcanic-hydrothermal CO2 and biogenic
fluxes that coexist and are of the same order of magnitude. The rate of CO2 emissions through an area
of 4 x 2.5 km was from 14162 to 23105 t y-1. The amount of CO2 fixed in bedrock was quantified by
coulometric carbon titrations of 50 drill cutting samples from 5 high temperature geothermal wells in
the southern region of Berlin. The amount of CO2 in the drill cuttings ranged between 0.0 and 194 kg
m-3. For wells located in the westernmost region the CO2 content reaches the highest values in the
uppermost 1200 m but it decreases sharply below that depth and very little CO2 is present in minerals
below 1500 m. A more homogeneous CO2 distribution has been observed in wells located in the
eastern part of the area studied where CO2 content of the order of 60 kg m-3 was found at 1845 m
depth. The load of the bedrock was computed to be 79.5 kg m-2 and the total amount of CO2 fixed in
the bedrock was estimated as 795 Mt. The CO2 fixation rate of the system was computed to be
between 6100 and 79500 t y-1. CO2 emissions from the power plant amounted to 42420 t y-1 in the
year 2008, double the natural emissions observed in the present study.