Relationship between CO_2 concentration and groundwater quality at the mouths of springs on a coral island (Miyako Island, Japan)
2011 Volume 82 Issue 4 Pages 275-282
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2011 Volume 82 Issue 4 Pages 275-282
We measured CO_2 concentrations and water quality at the mouths of four springs (one in a forested area and three in agricultural areas) of unconfined groundwater on Miyako Island, Japan, from March 2009 to March 2010. On this coral island, ammonium sulfate is used extensively, mainly for sugarcane fertilization. The results were as follows: (i) In the agricultural areas, the CO_2 concentrations were high (1722-3277 ppmv on average) and variable, whereas those in the forested area were low (526 ppmv on average) and stable. (ii) The amounts of CO_2 emitted from the spring mouths in the agricultural areas were about 3.6-4.8 times the amount emitted in the forested area. (iii) CO_2 concentration was positively correlated with Ca^<2+>, NO^-_3, and H^+ concentrations in the groundwater in the agricultural areas. (iv) In the agricultural areas, H+ concentration in the groundwater was positively correlated with Ca2+ and HCO3- concentrations; likewise NO3- concentration was positively correlated with K^+, Ca^<2+>, Mg^<2+>, SO^<2->_4, and HCO^-_3 concentrations. (v) In all the areas, the seasonal change in H^+ concentrations in the groundwater was positively correlated with the amount of nitrogen in the fertilizer used for sugarcane cultivation. Taken together, our results confirm that CO_2 at the spring mouths was derived from carbonate dissolution accelerated by H^+ produced during nitrification of ammonia. The SIc and PCO_2 values indicated that some of the carbon released from the dissolution of coral in the agricultural areas was carried in groundwater and eventually emitted at the spring mouths (indirect emissions). The CO_2 concentrations at the spring mouths in the agricultural areas could be expressed as a function of groundwater temperature and H^+ and Ca^<2+> concentrations, whereas the CO_2 concentration in the forested area could be expressed as a function of temperature alone.
