Suburban land use is expanding in many parts of the United States and there is a need to better understand the potential water-quality impacts of this change. This study characterized groundwater quality in a sandy, water-table aquifer influenced by suburban development and compared the results to known patterns in water chemistry associated with natural, background conditions and agricultural effects. Samples for nutrients, major ions, and isotopes of N and O in NO₃⁻ were collected in 2011 beneath turfgrass from 29 shallow wells (median depth 3.7 m) and from 18 deeper wells (median depth 16.9 m) in a long-term suburban development. Nitrate (as N) concentrations in groundwater beneath turfgrass were highly variable (0.02–22.3 mg L⁻¹) with a median of 2.7 mg L⁻¹, which is higher than natural water chemistry (>0.4 mg L⁻¹; Na⁺–Cl⁻–HCO₃⁻ water type), but significantly lower than concentrations beneath a nearby agricultural area (median 16.9 mg L⁻¹; p < .0001). Dissolved Fe concentrations in shallow suburban groundwater, attributed to chelated Fe in turfgrass fertilizers, were significantly higher (p < .005) than concentrations from the agricultural site, although a Ca²⁺–Mg²⁺–Cl⁻–NO₃⁻ water type was dominant in both areas. A Na⁺–Cl⁻–NO₃⁻ water type indicated a septic-system source for nitrate in deep suburban groundwater (0.06–6.0 mg L⁻¹; median 1.5 mg L⁻¹). Isotopic data indicated denitrification; however, geochemical techniques were more helpful in identifying nitrate sources. Results indicate that suburban expansion into agricultural areas may significantly decrease overall nitrate concentrations in groundwater, but excessive turfgrass fertilization could result in localized contamination.