The Hesbaye area is located in the northeastern part of Belgium. The aquifer formations consist of chalk deposits. Groundwater provides about 80,000 m 3 d -1 . Despite 5 to 20 meters of superficial loess deposits, the groundwater quality is threatened by increasing nitrate concentrations of 0.35 mg·L -1 per year in the semi-confined part of the aquifer to 0.7 mg·L -1 in the unconfined aquifer. Presently, nitrate concentrations are between 15 and 25 mg·L -1 in the semi-confined part of the aquifer but are more than 35 mg·L -1 (reaching locally 150 mg·L -1 ) in the unconfined part that covers 95% of the area. Nitrate concentrations have such a high spatial variation that various statistical treatments (such as kriging used to draw iso-concentration maps) have failed. This failure is due to the fact that the concentrations are highly influenced by surface land use (grass land, culture land, villages, point source pollutants, etc.). In addition, nitrate content in the aquifer varies vertically with decreasing values at depth (gradient of 0.7 mg·L -1 ·m -1 . Aquifer parameters were detemined by 38 pumping and tracer tests conducted in radial convergent or cylindrical flow at 11 sites. Results showed that hydraulic conductivity values ranged from 1 · 10 -6 m·s -1 to 4 · 10 -2 m·s -1 and effective porosities from 0.5% to 7%, showing that the aquifer was heterogeneous. Dispersivity values were affected by scale effects and varied according to chalk weathering or fracture zones. They ranged from less than 5 m in fractures to more than 60 m in weathered chalk (as in the upper part of the aquifer) and in the chalk matrix. In the chalk, transport processes were influenced by the immobile water effect due to diffusive transfer from the moving to the non-moving fluid. Non-effective porosity filled by non-moving fluid was estimated between 8 to 42%. The transfer constant ranged from 0.98 · 10 -7 s -1 to 10 · 10 -7 s- 1 . The determination of the transport parameters allowed simulation of nitrate transport at a regional scale. ...