The study area extends within the geographical limits of (13° 03’ 59.99” N, 80° 17’ 25.63” E and 12° 31’ 18.59” N, 80° 9’ 56.60” E) and a coastal stretch of about 170 km extending from north Chennai to the south of Puducherry along the southeast coast of India ( Figure 1 ). Sediment samples were collected at 11 transects (1 km, 5 km, and 10 km from the coast) such as Chennai Port (CP-1, CP-5, CP-10), Marina (MA-1, MA-5, MA-10), Adyar (AD-1, AD-5, AD-10), Kovalam (KO-1, KO-5, KO-10), Mahabalipuram (MH-1, MH-5, MH-10), Kalpakkam (KP-1, KP-5, KP-10), Edaikazhinadu (ED-1, ED-5, ED-10), Pondicherry University (PU-1, PU-5, PU-10), Gandhi statue (GS-1, GS-5, GS-10), Fishing harbor (FH-1, FH-5, FH-10), and Paradise beach (PB-1, PB-5, PB-10). The coast of Tamil Nadu has 3 major ports, 7 state ports, 16 minor ports, fishing ports, and various coastal industries such as nuclear power plants, thermal power plants, oil refineries, and fertilizer production. The megacity Chennai, the capital of Tamilnadu, has also experienced rapid industrial and population growth as well as seeing untreated domestic and industrial wastes discharged into the coastal waters (Rosado et al., 2023). Such coastal cities have an estimated population of around 20 million people (Ramesh et al., 2008). More than 10 million people live along the coast in the mega city of Chennai alone, resulting in more than 75 million gallons of waste being discharged into the sea every day (Ramesh et al., 2008). The Buckingham Canal, which is located within the study area, runs parallel to the coast and transports largely untreated sewage from the city and industrial effluents to the backwaters of the Cooum, Adyar, Muthukadu, Edaiyur, and Sadras. Additionally, the Madras Atomic Power Station (MAPS) on the Kalpakkam coast draws seawater through an intake structure that is submerged in water about 500 meters from the coastline. The hot seawater is discharged into the sea through an outfall canal after being heated by the condenser during the cooling process. The coast is characterized by a semi-diurnal tide of a maximum ~1.2 m tidal range. Coastal currents vary seasonally; with an average of 15 cm/s, northeasterly during the southwest (SW) monsoon and 17 cm/s in a southerly direction during the northeast (NE) monsoon. Five important tourist beaches viz., Marina (MA), Elliot (EL), Thiruvanmiyur (TH), and Kovalam (KO) in Chennai, and Paradise Beach (PB) in Puducherry, are located along the southeast coast of India.

A total of four scientific coastal cruises up to 10 km from the coast along the Chennai-Puducherry region were conducted during 2019-20. The research vessels of the Ministry of Earth Sciences, Govt. of India were engaged for these on-board samplings. Details of the cruises are: first cruise- Mar 2019 by CRV Sagar Purvi, second cruise-January 2020 by ORV Sagar Manjusha, third cruise-July 2020 by CRV Sagar Anveshika, and fourth cruise-September 2020 by CRV Sagar Anveshika. The four cruise data sets were separated into two seasons. Depending on the amount of precipitation (IMD, 2020), the seasons are classified into dry periods (January and March) and wet periods (July and September). A total of 132 coastal sediment samples were collected from the Chennai-Puducherry coast. After collection, the sediment samples were air-dried for one week, then ground using a porcelain mortar and pestle, sieved through a 63 µ size, and stored in polyethylene bags at ambient room temperature. All analyses were carried out in duplicate and the results were expressed as mean. Briefly, 0.2 g of the sediment samples were digested with 1 ml H₂O₂, 4 ml HNO₃, and 1 ml HCL in a 1:4:1 ratio. Samples were digested at 195°C in a microwave digestor for 90 min. The digested samples were then filtered through the Whatman No.1 filter and made up to 10 ml with Milli Q water (USEPA, 2007). Metal concentrations in extracted samples were measured by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Make: Agilent; Model: 5110 VDV). The instrument was calibrated using a trace metals mix (Agilent Technologies) standard. The certified reference materials (CRMs) for marine sediment (PACS-3, Lot G 4169010, NRC, Canada) were used to validate the analysis of heavy metals (Persaud et al., 1993; MacDonald et al., 2000; ECMDEQ, 2007; Simpson et al., 2013) ( Supplementary Table 1 ). The same acid digestion process used for the sediment samples was also used for CRMs. Sediment organic carbon (SOC) was determined using the Walkley-Black wet oxidation method (Walkley, 1935).

The pollution level and environmental quality status in marine sediments were assessed by using pollution indices such as enrichment factor (EF), contamination factor (CF), geo-accumulation index (Igeo), and ecological risk index (ERI). The background metal concentration is considered the standard value for reliable interpretation of the geochemical data. The average metal concentrations in sediments were measured following Turekian and Wedepohl (1961).

A popular normalization technique used to distinguish metals with natural variability from those with anthropogenic variability is the enrichment factor (EF) (Sinex and Helz, 1981; Selvaraj et al., 2004). To assess the anthropogenic impact on heavy metals in sediments, the EF for each element was determined as follows.

Where EF is the ratio of the relevant samples to background shale average values, and C_X and C_Al stand for the concentrations of metals X and Al, respectively.

According to Bam et al. (2011), EF is categorized as background concentration<1; depletion to minimum enrichment 1-2; moderate enrichment 2-5; significant enrichment 5-20, very high enrichment 20-40, and extremely high enrichment >40.

To measure the contamination in sediments from specific metals, the contamination factor (CF) was calculated using the following equation:

“C_background” in the equation above refers to the metal content in the sediments before any anthropogenic input. According to Hakanson (1980), CF<1 represents low contamination, 1< CF< 3 suggests moderate contamination, 3< CF< 6 denotes considerable contamination, and CF> 6 refers to high contamination.

The geoaccumulation index (Igeo) is the estimate of probable metal enrichment of metals in the sediments (Muller, 1979). The equation for the derivation of Igeo is provided here under,

Where B_n is the background concentration value for metal n and C_n is the concentration of metal n in the sediments (Turekian and Wedepohl, 1961), factor 1.5 is used to account for any variations in the background data due to lithological variances. Using the world average shale values, Igeo was effectively computed. Muller (1979) defined five levels of sediment pollution: non-polluted (Igeo< 1), very slightly polluted (1<Igeo< 2), slightly polluted (2<Igeo< 3), moderately polluted (3<Igeo< 4), highly polluted (4<Igeo< 5), and very highly polluted (Igeo> 5).

Overall, the ecological impact of metal contamination in the sediment is evaluated using the ecological risk index (ERI) (Hakanson, 1980). The ERI is calculated by the formula,

Where Trⁱ stands for the toxic response factor for a certain metal, Cf ⁱ for contamination of a chosen metal, and Ef ⁱ for the enrichment of a particular metal. The degree of ecological risk is quantified using the following categories of ERI:

ERI< 40, which indicates low ecological risk due to metal pollution; 40 ≤ ERI< 80, which indicates moderate ecological risk; 80 ≤ ERI< 160, which indicates Considerable ecological risk; 160 ≤ ERI< 320 - high ecological risk; ≥ 320 - very high ecological risk.

Geo accumulation index (Igeo) is a measure of metal deposit in the sediment in comparison with the contextual concentration of the specific metal. The Igeo values of the heavy metals are depicted in Figure 2A . The Igeo value of the surface sediments in the study area showed that Cr ranged from -2.6 to -1.3 (mean -1.9), Cu -4.4 to -2.9 (mean -3.6), Cd -0.1 to 2.2 (mean 1.1), Pb -4.8 to -2.0 (mean -3.2), Ni -3.8 to -2.2 (mean -2.9), Zn -4.2 to -2.8 (mean -3.3), and As from 0.4 to 2.4 (mean 1.6). The Igeo index categorized Cd and As under the slightly polluted category. The Igeo values of other metals such as Cr, Cu, Pb, Ni, and Zn were less than 1, so these metals are shown to be non-polluted ( Figure 2A ). The Igeo values obtained in this study are in agreement with the previously reported values in Chennai coastal sediments (Tholkappian et al., 2018). However, the Igeo value of Cd was >2 for sediments during the dry period (CP, KO, ED, FH), but the Igeo value of As was >2 during both dry and wet periods (CP, MA, MH, PU) locations, which indicates that these locations are slightly polluted. The heavy metals (Cd and As) pollution tends to be higher in the study area during both dry and wet periods, possibly due to anthropogenic inputs such as untreated sewage discharge, fertilizers, pharmaceuticals, paints, distilleries, cement clinker, refineries, and ship breaking. Similar observations are reported along the southeast coast of India (Ranjan et al., 2008; Sundaramanickam et al., 2016). The Igeo value of metals is in the following order: As >Cd >Cr >Ni >Pb>Cu ( Figure 2A ).

The EF values for heavy metal pollution in the sediment are shown in Figure 2B . The EF value of the sediment for Cr ranged from 0.3 to 0.6 (mean 0.4), Cu 0.1 to 0.5 (mean 0.2), Cd 1.4 to 24.3 (mean 8.1), Pb 0.1 to 0.4 (mean 0.2), Ni 0.1 to 0.4 (mean 0.2), Zn 0.1 to 0.3 (mean 0.2), and As from 2 to 10 (mean 5.1), Cd showed the highest EF value followed by As among the other metals ( Figure 2B ). As was classified as moderate to significant enrichment whereas Cd was moderate to very high, while Cr, Cu, Pb, Ni, and Zn were classified as minimal enrichment which corroborates Figure 2B . Cd showed very high enrichment during the dry period whereas, As showed significant enrichment during the wet period. Except for MH and KP locations, the EF value of Cd was >5, while As was >5 at CP, MA, ED, PU, and FH. The average EF for Cr, Cu, Pb, Ni, and Cd was less than 1, while As was between 2 and 20, indicating that these metals were depleted to minimum to moderate enrichment in the sediments. The EF results revealed that the higher values for Cd and As are primarily attributable to contamination from metal-based industries such as electroplating and metallurgy, as well as automobile exhaust (Karthikeyan et al., 2020) The average order of EF values of metals was Cd > As > Cr >Ni >Pb>Cu ( Figure 2B ). Similarly, Rubalingeswari et al. (2021) found higher enrichment of Cr and Cu based on EF values of more than 30 in the sediment samples of the Adyar estuary on the Chennai coast. This indicates the metal load to the coastal sediments from the highly contaminated estuaries in the Chennai coastal region.

CF values describe the level of metal contamination in the coastal environment. The CF values for heavy metal contamination in the sediment are shown in Figure 2C . The CF value of Cr ranged from 0.3 to 0.6 (mean 0.4), Cu 0.1 to 0.5 (mean 0.2), Cd 1.5 to 25.6 (mean 8.5), Pb 0.1 to 0.4 (mean 0.2), Ni 0.2to 0.4 (mean 0.2), Zn 0.1 to 0.4 (mean 0.2), and As from 2.1 to 10.5 (mean 5.4), Cd shows the highest CF value followed by As ( Figure 2C ). Cd and As were classified as considerable to high contamination, while Cr, Cu, Pb, Ni, and Zn were classified as low contamination in the study area. The coastal sediments are highly contaminated by Cd during the dry, whereas As was during the wet period. The CF of metals is in the following order: Cd > As >Cr >Ni >Pb>Cu ( Figure 2C ).

The ERI values for heavy metals in the sediment are shown in Figure 2D . The ERI value of Cr varied from 0.4 to 0.8 (mean 0.5), Cu 0.1 to 0.5 (mean 0.2), Cd 25.2 to 444.1 (mean 147.7), Pb 0.2 to 0.7 (mean 0.4), and As from 20.9 to 105 (mean 53.9), Cd shows the highest ERI value followed by As ( Figure 2D ). ERI classified the sediments as moderate to very high ecological risk from Cd during the dry period, while the coastal sediments are under moderate to considerable ecological risk by As during the wet period in the study area. This coastal region has low ecological risk from metals such as Cr, Cu, and Pb. The ERI of metals in the sediment samples are in the following order: Cd > As > Cr > Pb > Cu ( Figure 2D ). In a similar observation, an ERI value of less than 150 was reported in the southern tropical estuarine sediments of the Indian coast (Nishitha et al., 2022). These values indicate that the aquatic organisms are not at risk from these metals.

The correlation coefficient for Cr, Cu, Pb, Cd, Co, Mn, Ni, Zn, As, SOC, Sand, Silt, and Clay components are depicted in Table 2 . The sand showed strong positive correlation with Cd (r ^{2 =} 0.986), Mn (r ^{2 =} 0.983), Co (r ^{2 =} 0.828), and Zn (r ^{2 =} 0.702), while Silt and clay showed strong positive correlation with SOC (r ^{2 =} 0.936), Ni (r ^{2 =} 0.933), Cr (r ^{2 =} 0.811), Pb (r ^{2 =} 0.756), and Cu (r ^{2 =} 0.753). This implies that these metals bond better to finer sand particles than to larger sand particles (Sundaramanickam et al., 2016) The SOC was strongly correlated with As (r ^{2 =} 0.981), and Zn (r ^{2 =} 0.703). This suggests that the sediment organic matter and its properties act as metal binders, with SOC playing an important role in its distribution pattern (Samuel and Phillips, 1988; Sundaramanickam et al., 2016). Additionally, As was strongly positively correlated with Cr (r ^{2 =} 0.982), Co (r ^{2 =} 0.853), and Pb (r ^{2 =} 0.619). Mn substantially correlated with Cd (r^{2 =} 0.900) and Pb (r^{2 =} 0.707), whereas Co positively correlated with Pb (r^{2 =} 0.877), Cu (r^{2 =} 0.865), Cr (r^{2 =} 0.811), and Cd (r^{2 =} 0.634), which shown in Table 2 .