The Acu Valley is the lower part of the Rio Piranhas valley in the northwestern part of the State of Rio Grande do Norte, Brazil. It begins where the Rio Piranhas leaves the crystalline Precambrian rocks to flow across the outcrop of sedimentary rocks. The area considered in this report extends northward for about 45 kilometers; it is terminated arbitrarily where encroachment by sea water has contaminated the aquifer and imparted a disagreeable saline taste to the water in it. The boundary was not determined in the field, however, for lack of special equipment. Part of the extensive uplands on either side of the valley are included. This makes the total area approximately 2,500 square kilometers. The largest town, Acu, had a population of about 8,000 in 1960. The area is considered to be part of the Drought Polygon of northeast Brazil because the precipitation, although averaging 448 millimeters annually at Acu, varies widely from year to year and often is deficient for many months. The precipitation has been supplemented by use of irrigation wells, but irrigated agriculture is not yet far advanced, and the quantities of water used in irrigation are small. Geologically, the area consists of basement crystalline rocks (Precambrian), a wedge of sedimentary rocks thickening northward (Cretaceous), and alluvial sediments constituting a narrow band in the bottom of the valley (Alluvium and terrace deposits). The crystalline rocks contain water mainly in fractures and, in general, are impermeable. The sedimentary rocks of Cretaceous age comprise two units: a thick but fine-grained sandstone grading upward into siltstone and shale (Acu Sandstone), and limestone and dolomite with an included shale zone (Jandaira Limestone). The sandstone especially and the limestone to a lesser degree are ground-water reservoirs of large capacity. The limestone has been tapped at several places, but the sandstone and its contained water are practically untested and, hence, imperfectly understood. The alluvium of the first terrace is the aquifer supplying most of the ground water being used in the area. Wells in the alluvium yield as much as S0,000 liters per hour. Larger yields probably could be obtained from wells designated to take full advantage of the aquifer. There are in the valley about 300 dug wells which are used for irrigation. Half of these are equipped with pumps and engines. The rest, together with about 500 drive-point wells, are equipped with manual or windmill-driven pumps. In addition to irrigation, the water is used in homes and for cattle. The quantities of water currently used in irrigation are relatively small, both per hectare and in the area as a whole, but .this will probably increase substantially when intensive irrigation becomes a reality. The annual pumpage from the alluvium, nearly constant since 1959, was about 2.5 million cubic meters in 1964, which is only about 90 cubic meters from each hectare-meter of saturated alluvium. This amount would lower the water table about 1 meter in 11 years, if there were no recharge. Actually, no such decline is likely to occur, because the recharge from precipitation alone is estimated to be more than enough to replace the water currently being pumped. Chemical analyses of eight samples show that the ground water in the alluvium is acceptable for most uses. The water in the Acu Sandstone and Jandazra Limestone is more mineralized than that in the alluvium and at some places, at least, is not acceptable for human consumption. The available chemical data on this water, however, are not adequate to judge fully the quality of the water in these formations. It is estimated that about .'22 million cubic meters of water would be needed annually if irrigation were extended to all the bottom land, which totals about 25,000 hectares. This amount is only one-fourth to one-half the estimated recharge from precipitation alone. The present rate of application of water is very low