# Three-Year Follow-Up Assessment of Anthropogenic Contamination in the Nichupe Lagoon
## 3 Results and Discussion  
Descriptive statistics of caffeine and nutrients in the Nichupte Lagoon System are detailed in Table 2.  
The presence of caffeine was detected in all study zones of NLS, which show an increasing tendency in all sites in 2019 with peak concentrations of 2880 ng L\({}^{-1}\) and 425 ng L\({}^{-1}\) in Zone I (Bojorquez) and Zone II (North), respectively (Figure 2). But when caffeine (average value) is plotted against the number of tourists, it is appreciable that it does not fully account for what was found, since 2018 had tourists like 2019 who had a median caffeine value like the one in 2020 that had c.a. half of the tourist. This is an unexpected outcome, but it could be confirming what other [26; 27] mention regarding the dependance between pollutants transport/dispersion and hydrological changes related with the anomalous precipitation events. Figure 2 also shows the precipitation in the area, and for 2020, according to CONAGUA, there was an anomalous precipitation (4\(\times\) the common one of 100 mm) and high precipitation (2\(\times\) the common one) in 2018 [28], both of which were prior to the sampling campaign (August); this could have promoted a higher hydraulic head that increased the dilution of caffeine in the groundwater system. For 2019, almost no rain was prior to the sampling; this diminished the hydraulic head, promoting less dilution and less transport, allowing a longer residence time of the wastewater and caffeine local inputs. Thus, when caffeine results are integrated to the hydrological process of the area, it could be seen that they are not only reflecting the presence of wastewater in the groundwater system, but also how it could be altered by the previous precipitation events. Figure 2: Average annual variation in the concentration of caffeine and number of tourists (**left** side) and monthly precipitation in Cancun 2018–2020 (**right** side). Furthermore, rainfall and the connectivity of the submarine springs to the nearshore coastal environment contribute to the transfer of caffeine from the inland zones to the NLS. These discharges of groundwater at the Mexican Caribbean range from 48 m\({}^{3}\) d\({}^{-1}\) m\({}^{-1}\) in the northeast region to 568 m\({}^{3}\) d\({}^{-1}\) m\({}^{-1}\) in the southeast areas [12]. Moreover, the NLS dynamics is also influenced by processes such as tides, waves, winds, and the geometry and bathymetry of the system [17], provoking restricted circulation patterns (see Figure 1) that allow the accumulation of caffeine under dry or low precipitation conditions. This could explain why the highest concentrations of caffeine in 2019 were in Z1 Bojorquez (2880 ng L\({}^{-1}\)), Z1 North (425 ng L\({}^{-1}\)), and Z5 Rio Ingles (85 ng L\({}^{-1}\)), as is seen in Figure 3.  
Most of the caffeine values detected in this study are within or above the concentrations ranged from non-detected to 2390 ng L\({}^{-1}\), reported by [22], a study also conducted in coastal lagoons near urban and touristic settlements in the Yucatan Peninsula. Moreover, the seasonal increase in population due to tourism in the coastal areas has been correlated with a significant increase in caffeine concentrations in coastal waters [29]. In the Cancun area alone, the tourism rate is measured through hotel occupancy and the number of tourists per year, and an average occupancy of 75% was reported during 2018 and 2019 with more than 7 million visitors per year [24], compared to the drastic decrease in these numbers and consequently, in caffeine concentrations in 2020 during the COVID-19 pandemic; as shown before, the hydrological conditions effect on the groundwater system also play an important role in the caffeine concentrations. Since the Nichupte lagoon is surrounded by three main urban areas, the hotel area on Cancun Island, the first phase of the City of Cancun, and the area where the international airport is located, there is, therefore, a consequent unplanned urban expansion that takes place where the treatment of water sewage is inadequate [30].