## 3 Results and Discussion
### Sediment Characterisation  
The grain size of sediment samples (SI, Figure S1) was predominantly coarse and very coarse sand (>60%). The least representative fractions were silt and mud (<0.23% for PA, <0.05% for PB and PC). The chemical composition of sediments in the different stations, plus average values and previous data collected, are summarised in Table 2. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **PA** & **PB** & **PC** &
\begin{tabular}{c} **Marine** \\ **Surface Water** \\ **(Acute/Chronic)** \\ \end{tabular} \\ \hline \(T\) & \({}^{\circ}\)C & 15.66 & 16.43 & 15.02 \\ \(pH\) & 7.68 \(\pm\) 0.01 & 7.65 \(\pm\) 0.01 & 7.53 \(\pm\) 0.005 \\ \(S\) & psu & 39.84 \(\pm\) 0.03 & 39.60 \(\pm\) 0.14 & 39.93 \(\pm\) 0.001 \\ \(EC\) & \(\mu\)S cm\({}^{-1}\) & 59.431 \(\pm\) 40 & 59,093 \(\pm\) 194 & 59,580 \(\pm\) 10 \\ \(TDS\) & ppt & 29.71 & 29.54 & 29.79 \\ \(El\) & mV & 140 \(\pm\) 1.14 & 204 \(\pm\) 2.21 & 217 \(\pm\) 0.21 \\ _Alk_ & mgCaCO\({}_{3}\) L\({}^{-1}\) & 216 \(\pm\) 32 & 229 \(\pm\) 67 & 185 \(\pm\) 17 \\ _Element concentrations_ & Ag & mg L\({}^{-1}\) & bdl & bdl & bdl \\ Al & \(\mu\)g L\({}^{-1}\) & 106 & 670 & 713 \\ As & \(\mu\)g L\({}^{-1}\) & bdl & 1.60 & 1.75 & 69/36 \\ Ca & mg L\({}^{-1}\) & 377 & 326 & 371 \\ Cd & \(\mu\)g L\({}^{-1}\) & 0.22 & 0.37 & 0.21 & 40/8.8 \\ Cu & \(\mu\)g L\({}^{-1}\) & 15.1 & 1.7 & 3.5 & 4.8/3.1 \\ K & mg L\({}^{-1}\) & 359 & 310 & 355 & -/100 \\ Li & \(\mu\)g L\({}^{-1}\) & 130 & 163 & 166 & \\ Mg & mg L\({}^{-1}\) & 1207 & 1016 & 1241 & \\ Mn & \(\mu\)g L\({}^{-1}\) & bdl & bdl & 3.40 & \\ Na & mg L\({}^{-1}\) & 9946 & 8722 & 10,200 & \\ Pb & \(\mu\)g L\({}^{-1}\) & bdl & 8.3 & 40 & 210/8.1 \\ Zn & \(\mu\)g L\({}^{-1}\) & 33.3 & 46.6 & 60.0 & 90/81 \\ \hline \hline \end{tabular} bdl: below detection limits.  
\end{table} 
Table 1: Physical-chemical parameters in the water at each sampled station (PA, PB, PC), average element concentration in seawater (N = 3) and marine reference exposure limits according to NOAA [29]. Cd concentration in sediments was below the detection limit for the northern stations, while 1.11 mg kg\({}^{-1}\) was found in PC; this value was above the concentrations previously seen in the area (0.095 mg kg\({}^{-1}\)) (Table 2). Al concentration in sediments ranged between 1850 and 2247 mg kg\({}^{-1}\); these concentrations were below values (4070 mg kg\({}^{-1}\)) previously reported in the area. Concentrations of Fe in sediments were similar to those found along the coast (3813 \(\pm\) 271 mg kg\({}^{-1}\)). There was a significant accumulation of Cu in sediments from the National Park (PA, 9.31 mg kg\({}^{-1}\)), which surpassed the average concentration in the area (Table 2), although there is a massive accumulation in Chanaral from historical mining residue discharges (7.20-985 mg Cu kg\({}^{-1}\)) [30; 31]. The Pb concentration in sediments varied in the study area, ranging between 7.67 and 27 mg kg\({}^{-1}\): PA (21.30) \(>\) PC (17.17) \(>\) PB (10.66). Nevertheless, these are normal concentrations in the area; a previous study [30] found values from 1.57 to 51 mg Pb kg\({}^{-1}\). The same tendency was found for As, where averaged concentrations (39.88 \(\pm\) 18 mg kg\({}^{-1}\)) agreed with previous studies [32; 33], who found concentrations between 38.10 and 117 mg kg\({}^{-1}\). However, a significant difference (\(p<\) 0.05) was found between concentrations in sediments from PB (60.25 mg As kg\({}^{-1}\)) and PC (19.73 mg As kg\({}^{-1}\)). The concentrations of As surpassed the threshold effect levels (TEL), i.e., the concentrations had a certain probability of being toxic as tested through standard bioassays. The concentration of Zn in sediments from PC was significantly different (\(p<\) 0.05) for PA, but none of the concentrations surpassed the TEL values.