Biomonitoring is a widely used technique to determine the condition of natural water quality, through the study of aquatic macroinvertebrates as biological indicators. This study aimed to estimate the water quality of the Arenal-Tempisque (DRAT) irrigation district, using the biomonitoring technique. Macroinvertebrate were sampled every seven weeks times at nine sites, using the standardized method with a D-net, and water samples were taken every six months. Once all the material was identified by taxonomic family, the BMWP index and the Shannon-Wiener diversity index were calculated. Additionally, statistical analyses of conglomerates and principal components identified in the physicochemical and biological data, as well as a combination of these, both in wet and dry periods, were determined. A total of seven samplings for macroinvertebrates were conducted and 4559 individuals were identified and grouped into 36 families in 16 different orders. According to the BMWP index modified to Costa Rica, the quality of surface water is poor and is contaminated with organic material. However, the water quality is adequate for its use in agriculture, according to the physicochemical analysis. A comparison was made between years and seasons (dry, wet) and no change in water quality could be detected. According to the Shannon-Wiener diversity index, the biodiversity at the sites is meager. An analysis of physicochemical and biological data revealed that families Gomphidae, Corydalidae, Physidae, and Tubificidae can be used as bioindicators, as well as Ca, water hardness, DQO, DBO₅ as physicochemical parameters to supplement biomonitoring.