In this work, 16 samples of underground water in Al-Qassim region of Central Saudi Arabia were chemically, physicochemically and biologically investigated. Quantitative determination of Ca, Mg, Na, K, (SO₄)^2-, (CO₃)^2-, along with EC and pH measurements. In addition, microbiological activity of all water samples was also investigated. The results clearly show the very high EC values for 50% of the water samples reaching a maximum of 16.66 mmhos/cm indicating potential irrigation problems affecting both plant and soil. However, the other part of the samples have reasonable EC values ranging from 1.01 to < 4.0 mmons/cm. The very high EC values were discussed in terms of high ionic concentration. No coliform bacteria was detected in any of the samples indicating the absence of mixing between sewage water and underground water.