Introduction Nitrate contamination is responsible of several diseases as hypertension, cancer and birth defects (Spalding and Exner 1993) thereby, a maximum of 45 mg/l for drink- ing purposes is worldwide accepted (WED 2001). As in other parts of the world, there is evidence that NO 3 ) concentrations in many aquifers of Japan are assuming serious dimensions as a direct consequence of the intensification of agricultural activities (Ii et al. 1997; Mohamed et al. 2003; Tase 2004). However, despite being the most densely populated and economically most productive district of Japan, very little is known about the conditions in shallow groundwater within the agricultural belt surrounding the Tokyo metropolis. Contamination in this area causes particular concern because of its proximity to urban centers, and the important number of people still relying on groundwater for water supply. The lack of information emphasizes the need to urgently evaluate the sources and extent of NO 3 ) pollution in the region in order to develop the correspondent control and management strategies. As the movement of solutes in groundwater is a three- dimensional process influenced by a multitude of vari- ables with complex functions and interactions, it seldom can be overcome even by an extensive monitoring net- work, so a simulation model may provide important additional information (Uffink and Ro¨mkens 2001). Although the model results are influenced by a relative uncertainty, they provide a framework for synthesizing field information and for testing ideas about how the system works (Woessner and Anderson 1994). Thus, the present research complements geochemical information with a finite-difference numerical simulation with the objective of generating quickly and at a relatively low cost, a first base model, which can help the authorities in Adrian H. Gallardo Walter Reyes-Borja Norio Tase Flow and patterns of nitrate pollution in groundwater: a case study of an agricultural area in Tsukuba City, Japan Received: 15 May 2005 Accepted: 17 June 2005 Published online: 2 August 2005 Ó Springer-Verlag 2005 Abstract A numerical simulation was applied to first characterize the groundwater flow and patterns of nitrate pollution of a small-agricul- tural catchment in Tsukuba City, Japan, for a 10-year period. There was a good performance of the flow simulation. In contrast, although the transport model calculated the evo- lution of the plume, it only provided estimates of solute concentrations. Groundwater contamination in- creased exponentially during the first 594 days of the simulation, reaching then a near-equilibrium state. Fer- tilizer applications are responsible for most of the leaching of NO 3 ) to groundwater, therefore, shifting of crops and the associated agricultural practices may translate into de- creases of contamination levels. A series of hypothetical scenarios demonstrated that replacing grass- lands by other crops may reduce the contamination levels up to 12%. As the chosen field is a representative of many other agricultural areas in Ja- pan, the approach and results should also be applicable to similar cases around the country. Keywords Groundwater contamination Æ Nitrate Æ Agriculture Æ Fertilizers Æ Tsukuba City Æ Ibaraki prefecture Æ Japan Environ Geol (2005) 48: 908–919 DOI 10.1007/s00254-005-0029-8 ORIGINAL ARTICLE A. H. Gallardo (&) National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan, 1-1-1, #7 Higashi, Tsukuba 305-8567, Japan E-mail: ad.gallardo@aist.go.jp Tel.: +81-29-8613240 Fax: +81-29-8613240 W. Reyes-Borja Æ N. Tase Æ A. Gallardo School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Ibaraki 305-8572, Japan