Abstract
This study assesses the water quality of Gomaspan Dam (GD), a recently commissioned impoundment on the Bastora River located nearly (30–35) km northeast of Erbil in the Kurdistan Region of Iraq. Field monitoring was conducted at nine sites (S1–S9) selected to represent inflow-proximal zones, mid-reservoir pelagic waters, near-dam/outlet, and shorelines influenced by settlement/agricultural activity. General physicochemical variables were measured across seven sampling months (October 2024–April 2025), while the macro/trace elements were assessed for two months during the study (December 2024 and April 2025) using Inductive Coupled Plasma (ICP) at a certified laboratory. In parallel, remote sensing in Google Earth Engine was used to map and summarize selected water-quality indices (e.g., turbidity patterns via Normalized Difference Turbidity Index (NDTI), chlorophyll dynamics via Normalized Difference Chlorophyll Index (NDCI), and transparency via Secchi disk depth (SDD) products, Total Nitrogen (TN) and Total Phosphorus (TP)), producing monthly outputs and longer-term composites.
The results indicated that several core parameters relevant to general water mineralization and the oxygen status remained within commonly cited guideline thresholds during the study period. The pH remained within the 6.5–8.5 criterion, dissolved oxygen met the ((5 mg/L) benchmark throughout (with February approaching the lower bound), total dissolved solids stayed below 500 mg/L, and BOD5 remained within the cited surface-water benchmark ((3 mg/L). When using the integrated Water Pollution Index (WPI), the reservoir yielded WPI = 0.48, placing GD in the “excellent water” class under the adopted classification. Turbidity exceeded the 5 NTU benchmark in multiple months and at specific site (notably S3).
Nutrient concentrations were consistently above cited Environmental Protection Agency (EPA) ecoregion reference-condition ranges, with TP monthly means reported at 0.06–0.72 mg/L and TN frequently elevated relative to reference ranges (site means reported at 1.85–2.16 mg/L), implying an increased risk of eutrophication pressures as the reservoir continues to stabilize. Coliform indicator levels Most Probable Number (MPN) exceeded the listed mean equivalent in several months and at some sites, although remaining below the statistical threshold equivalent value.
The trophic condition assessment, implemented using a Carlson-type index framework based on TP, TN and transparency, produced Trophic State Index TSI(TP) = 67.3, TSI(TN) = 42.0, and TSI(SDD) = 32.2, with an overall mean TSI ≈ 47.2. This pattern indicates a mesotrophic overall condition, but with an important internal imbalance: phosphorus conditions aligned with eutrophic potential while nitrogen and transparency suggested lower trophic expression at the time of sampling. The TN:TP mass ratio (~5.3) further supports the likelihood of nitrogen limitation, meaning that algal biomass may not yet fully track the elevated phosphorus signal, although the system may be vulnerable to shifts under changing loads and stratification regimes.
Trace-element screening showed generally limited detections in December 2024, with several metals not detected across sites, whereas April 2025 exhibited broader detectability for some elements (notably arsenic and copper) and localized maxima, including a pronounced Cu concentration at site 1 (0.642 mg/L) and repeated multi-element elevation at site 9.
Remote sensing results complemented field observations by revealing strong seasonal structure and persistent spatial organization. Sentinel-2–derived transparency Secchi Disc Depth (SDD) ranged from 3.99 to 9.72 m (mean 6.86 ± 1.52 m), with clearer conditions in October–November 2024 and reduced transparency in April 2025; longer-term composites showed lower clarity concentrated near margins and likely inflow/nearshore zones, consistent with sediment and/or pigment influences. Turbidity patterns from NDTI and chlorophyll tendencies from NDCI similarly emphasized shoreline/embayment hotspots and seasonal pulsing; importantly, the thesis notes that index-based mapping supports robust spatial/temporal comparison.
Multivariate analysis (PCA) further indicated that seasonal temperature shifts, nutrient availability, and ionic composition jointly structured variability, with warm months clustering with higher temperatures (and increased TP), while cooler months grouped with higher TN and stronger mineral/ionic signals.
GD exhibited excellent overall quality for the assessed major physicochemical indicators (WPI = 0.48), yet key management risks are evident in the form of (i) recurring turbidity exceedances, (ii) nutrient enrichment above reference-condition ranges, (iii) intermittent elevation of microbial indicators, and (iv) seasonally enhanced detectability of some trace elements.
