###### Abstract  
The contamination of the Lower Danube Basin with trace elements has been the subject of intense study in recent years, with several pollution indices being employed for the assessment of water, soil, and sediment quality. Most pollution indices are calculated by comparing the measured concentration to a reference value, with different values from various sources being used. In this study, the threshold values for trace elements (As, Cd, Cu, Hg, Ni, Pb, and Zn) in Romania were selected based on the median of their concentration in stream sediment from 167 locations, which were carefully selected to be far from contamination sources. The threshold values were established based on a calculation starting from the geochemical concentration presented in the Geochemical Atlas of Romania (scale: 1:3,000,000, developed in 2006 by a collaborative Romanian-German team). By using these thresholds, pollution indices were calculated for nine sampling locations, from which suspended sediment, bottom sediment, and active floodplain sediment samples were collected and analyzed in an accredited laboratory. Most of the selected thresholds were found to be below the limits for sediments established by Romanian legislation, except for Cr and Ni, the higher values of which are consistent with the specific geological background of the country and the broader Balkan region. 2025 15 2458 2025  
The quality of the Danube River water and its sediment can impact the health of humans and ecosystems in the region. A substantial body of research has been conducted to gain insight into the river's geomorphology [3; 4] and erosion and sedimentation processes [5], as well as the impact of hydropower dams [6; 7], navigation [8; 9], agriculture [10; 11], industry [12], mining [13; 14], waste disposal [15; 16], and other anthropogenic activities on the river [2; 17]. The Third Joint Danube Survey (JDS3), conducted by the ICPDR along the entire length of the Danube River, revealed that contamination with trace elements was the primary source of acute and chronic toxicity in benthic invertebrates at the sampled stations, and industrial waste was the second-most significant contributor to this phenomenon [18; 19]. The analysis identified several chemical substances, including insecticides, herbicides, fungicides, pharmaceuticals, home and personal care products, and other such chemicals, which were found to manifest chronic toxicity with low or no detectable toxic effects (in the latter case, the substances were not detected). Trace elements have been demonstrated to possess carcinogenic properties when introduced into the human body [20]. As a result, there is a clear rationale for monitoring their concentration, identifying areas with elevated and potentially harmful concentrations, and implementing remediation strategies. Various risk assessment methods have been employed to accurately assess the degree of contamination with trace elements in riverine and estuarine systems. Among these, pollution indices have demonstrated efficacy as suitable tools [21]. A review of 18 of the most used indices in various studies worldwide [21] revealed that almost all were based on comparing the measured concentrations of trace elements against a set of reference values. These reference values are defined as geochemical background [22], pre-industrial values [23], average content in the Earth's upper continental crust [24], and average content in soil surface horizons worldwide [25]. Proportionally, a smaller number of indices are based on toxicological studies. These employ the tolerable levels in soil [26], meaning the concentrations at which there are almost no effects on benthic organisms (Effects Range Low, ERL) and at which probable effects occur (Effects Range Median, ERM) [27] or they are based on the ranks given to substances by the Agency for Toxic Substances and Disease Registry (ATSDR) [28].