This  paper  examined  the  influence  of  geology  on  the  groundwater  quality  in  the  Federal Capital  T erritory ,  Abuja,  Nigeria.  The  work  involved  the  study  of  fifty  (50)  groundwater samples  collected  by  the  National  Water  Resources  Institute,  Kaduna  across  the  six  Area Councils  during  its  “Assessment  of  Drinking  Water  Quality  in  Federal  Capital  Territory (FCT)”  in  2016. The result  of the  analysis of each  sample  was discussed in  terms  of the  rock  type  that  forms  the  aquifer  which  enabled  the  characterisation  of  the hydrogeochemical  facies  of  the  groundwater  and  the  mechanism controlling  its  quality . The  potential  health  risks  associated  with  ingesting  heavy  metals  in  water  were determined. The main hydrogeochemical facies  is  the  mixed type  of Ca-Mg-Cl and NaHCO -Cl  with none of the  cation-anion pair being dominant in  the  groundwater .  The 3 dominant mechanism controlling the  quality of groundwater in  the  study area is  rock weathering followed by the chemistry of rainwater .  The hazard index ranges in  value  between  0.0  and  0.6.  Since  the  maximum  value  is  less  than  1,  the  ingestion  of  the groundwater  in  the  study area  poses  no  significant  risk  of  non-carcinogenic  effect  to  the consumers.

KEYWORDS:  FCT ,  groundwater  quality ,  hydrogeochemical  facies,  heavy  metals,
health  risks