Arsenic (As) presence in groundwater from Argentinean Pampean plain is favored by high pH and Na-HCO 3 − https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781351046633/8978195a-ffaa-498f-8e06-bee4769ddb72/content/eq171.tif"/> type waters. Our hypothesis was that by adding Ca to the aquifer system, pH could decrease due to calcite precipitation and As may be retained in solid phase (sediments). Batch experiments were performed on natural sediments in contact with a Na-HCO 3 − https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781351046633/8978195a-ffaa-498f-8e06-bee4769ddb72/content/eq172.tif"/> solution, where CaSO4 or CaCl2 were added. Ca salts addition decreased down to 0.3 points of pH solution and as a result, As concentration in solution decreased. Two mechanisms are proposed. One is that a small decrease in pH enhanced As adsorption on Fe oxides. The second is that calcite precipitated and could trap As inside its structure.