To systematically investigate heavy metal contamination in groundwater of the Dagu River Basin, Qingdao, a total of 69 groundwater samples were collected from 23 wells along the river in January, March, and May, 2019. Concentrations of heavy metals—including aluminum (Al), arsenic (As), chromium (Cr), cadmium (Cd), nickel (Ni), copper (Cu), zinc (Zn), and lead (Pb)—and associated physicochemical parameters were determined. Groundwater pollution was assessed using the Heavy Metal Pollution Index (HPI) and the Nemerow Pollution Index (NPI). Potential sources of heavy metals were analyzed through Pearson correlation and principal component analysis (PCA). The fluxes of heavy metals transported by groundwater to Jiaozhou Bay were quantified, and historical data were compiled to compare spatial distribution patterns. Results show that the mean concentrations of all heavy metals were below the Class Ⅲ limits of the Chinese Groundwater Quality Standard (GB/T 14848—2017), although localized Zn exceedances occurred in March, and As and Ni exceeded limits in March and May at some sites. HPI and NPI assessments indicate that overall heavy metal contamination in the Dagu River Basin is low, with most sites uncontaminated, and a few downstream sites exhibiting light to moderate pollution, primarily contributed by Ni and As. Correlation and PCA results suggest that agricultural and aquaculture activities are the main sources of As, Cu, Al, and Cr, while industrial activities significantly contribute to Cd and Ni. Groundwater is an important source of Zn and As to Jiaozhou Bay. Compared with other regions, heavy metal concentrations in the Dagu River Basin groundwater are at an intermediate national level. This study provides essential data and scientific support for groundwater pollution prevention and management in coastal zones.