Iraq contains numerous oil fields, from which crude oil occasionally mixes with groundwater either directly or due to leakage from oil pipelines. Consequently, hydrocarbon pollution is caused, which is evident in the irrigation wells at the Desert Research Station in Najaf Governorate, Iraq. This study’s primary aim was to assess the extent of this pollution and implement a treatment process using locally available materials. In this study, wheat stalks collected from the surrounding area were used to remove oil and grease from contaminated groundwater. A simple physical treatment device was designed, manufactured and operated according to specific parameters to assess adsorption efficiency. Several factors affecting the adsorption process, including pH values (6.5, 7, 7.5, 8 and 8.5), retention time (15, 20, 30, 45 and 60 minutes) and adsorbent layer thickness (3, 5, 7 and 10 cm), were tested. The results showed that the highest oil and grease removal efficiency occurred at pH 8.5, with a thickness of 10 cm and a treatment time of 60 minutes, resulting in a significant reduction in pollutant concentrations. The removal efficiency reached 98.8%. The study also demonstrated that wheat straw is an effective and sustainable adsorbent for treating groundwater contamination in arid regions. Furthermore, the findings highlight the potential for applying low-cost, environmentally friendly treatment solutions for water remediation, which could be utilised in other areas facing similar pollution challenges.