Plastic Pollution in Wadis: Investigation at a Hotspot in Wadi Hoqain, Oman

1.1. General

Plastic pollution is recognized as a global problem. In 2022, Heads of State, Ministers of Environment, and other representatives from UN Member States endorsed a historic resolution at the UN Environment Assembly (UNEA-5) in Nairobi to end plastic pollution (CIEL, 2025). A total of 175 nations agreed to develop a legally binding agreement on plastic pollution by 2024. This agreement is currently still under discussion (Global Plastic Treaty, 2024).

Since its inception in the 1950s, global plastic production has increased exponentially, along with plastic pollution (Geyer et al., 2017), currently peaking at more than 400 million tons per year (Kubikova and Rudy, 2024). As a product with numerous convenient properties, plastic is produced and consumed in ever-increasing amounts, not only in industry and households, but also in the construction, transportation, and packaging sectors. However, its persistence in natural environments poses a significant problem, as plastic hardly degrades once it enters these ecosystems.

The abundance of plastic in the environment is widely recognized as a serious problem for ecological systems and human health. Plastic pollution not only destroys the natural beauty of terrestrial and aquatic environments but also poses a threat to native fauna and flora. Micro- and nanoplastics enter the food chain and consequently reach humans. In a recent review, Li et al. (2023) described the potential health impacts of microplastics, their environmental distribution, human exposure, and toxic effects. Khan et al. (2024), focusing on the pathway from fish to humans, highlighted the risks microplastic pollution poses to both fish and human health. Ullah et al. (2023) provided evidence of endocrine-disrupting effects in mammals.

Plastic pollution occurs when plastic waste is mismanaged, that is, not recycled, incinerated, or securely stored in sealed landfills. Geyer et al. (2017) estimate that, of all plastic ever produced, 30% is still in use, 12% has been incinerated, and only 9% recycled. More than half of all plastics produced to date have been discarded, accumulating in landfills or the natural environment. For 2019, Ritchie (2023) estimates that 22% of all plastics were mismanaged, including materials burned in open pits, dumped into seas or open waters, or disposed of in unsanitary landfills and dumpsites.

Studies on plastics in the environment currently attract significant attention, not only in political debates, the press, and among the public, but also within the scientific community (He et al., 2020). Research on marine ecology outnumbers studies on terrestrial systems by more than two to one. Furthermore, most studies focus on moderate or wet climates, with very few addressing the unique conditions of arid and semi-arid regions. Regarding Oman and the Oman Sea, only a limited number of publications explore various aspects of plastic pollution. In the Gulf of Oman, Khordagui and Abu-Hilal (1994) identified the potential threat of plastic pollution to the local ecology. Aliabad et al. (2019) highlighted a ‘pressing need to investigate the distribution of microplastics in sediments and biota as well as their effects on marine life and human health,’ a concern reaffirmed more recently by Kor et al. (2020). Additionally, in the mountainous region of Jebel Akhdar, Oman, Al-Mahrouqi and Victor (2017) found plastics to be the largest waste category.

To the best of the authors’ knowledge, no study on plastics in wadis has been published to date. With our work and tremendous support from the students of our university, we aim to fill this gap. A manuscript presenting our findings on plastics and microplastics on Omani beaches is currently in preparation (Ajaj et al., 2025). The primary focus here is on macroplastics in Omani wadis. While we have examined several wadis, including Al-Khoud and Shab, field excursions to Wadi Hoqain yielded the most interesting results. First, we describe the methods used in our field studies, followed by a presentation of the key findings.

1.2. Plastic Waste in Oman

There are no direct data on the amount of plastic waste in Oman; however, estimates can be made based on total waste generation and the proportion of plastic content. According to Al-Harthi (2015), Oman generated 1,190 kilotonnes (kt) of solid waste in 2010. Be’ah, the Oman Holding Company for Environmental Services, is a government-owned entity responsible for implementing waste management policy and effectively holds a nationwide monopoly in this sector. In its 2022 annual report, Be’ah reported waste collected through service providers at 1,432 kt in 2021 and 1,591 kt in 2022, alongside 478 kt in 2021 and 491 kt in 2022 collected from government entities and private compounds (Be’ah, 2022). This amounts to a total of 1,910 kt in 2021 and 2,002 kt in 2022, aligning with the approximately 2 million tonnes of municipal waste stated on Be’ah’s website (Be’ah, 2024). According to the same source, an additional 1.5 million tonnes of industrial waste should be included in the total estimate, while medical waste, at around 4.5 kt per year, contributes only marginally.

Be’ah reports that plastic accounts for 21% of the total waste (Be’ah, 2020). In a study of Al Jebel Al Akhdar, Al-Mahrouqi and Victor (2017) found plastic percentages of 26.7% for municipal waste, 30.4% for military waste, and 15.3% for waste from private companies. The authors note that the municipal figure ‘is not significantly different from the percentage (24.3%) reported by a landfill operator.’ For 2013, Al-Harthi (2015) reported a 32% share of plastic in the total solid waste collected by Be’ah. Assuming a 21% plastic share of the total waste, estimated at 3,500 kt per year, results in approximately 730 kt per year of managed plastic waste across Oman.

2.1. Hydrological Situation

For the field investigation, the waterfall within Wadi Hoqain was selected (GPS coordinates: 23.55000291, 57.34695567) (Fig. 1a-c). This waterfall is a popular local tourist destination in Oman, accessible from the capital, Muscat, in less than an hour by car. Due to visitors frequently leaving their garbage behind, the site has become a hotspot for waste pollution (Fig. 2a, b).

Within Oman, Wadi Hoqain is relatively favorable in terms of stream discharge, as the waterfall never completely dries up. However, in most years, discharge varies widely, from a mere trickle to a flood. Unfortunately, no hydrograph is available, nor is there a meteorological station at the site. The closest station is in the nearby regional town of Rustaq, where meteorological data are collected. The 12-year average there indicates 25 rainy days per year, irregularly distributed, with a maximum of 5 rainy days in April and a minimum of zero in September. The mean monthly precipitation is 18 mm, which is relatively high for Oman. Monthly rainfall peaks in April with an average of 41.9 mm (Worldweatheronline, 2024). Analysis of quarterly rainfall data from 2009 to 2024 shows a recurrence interval of 8 months for above-average rainfall events, while the recurrence interval for monthly rainfall exceeding 34 mm is approximately 2 years.

2.2. Field Campaigns

Two field trips to the selected location were conducted on February 27th and March 4th, 2023. During the period from January to March that year, only four days recorded rainfall, totaling 4.26 mm (Worldweatheronline, 2024). Consequently, no significant discharge events occurred, as illustrated in Fig. (2). Since a few garbage items had been washed away for over two months, accumulated pollution was clearly visible. Visual inspection identified the waterfall area as a major hotspot for garbage dumping. It was also evident that less plastic originated from upstream of the site. Downstream from this hotspot, 11 sampling sites were selected along the wadi. At each site, a designated area was marked where all macroplastics were collected, separated, classified, and counted (Fig. 3a and b).

Research on plastics classifies objects based on size: items smaller than 5 mm are termed microplastics, while larger items are classified as macroplastics. Mismanaged primary microplastics enter the environment directly after use, whereas secondary microplastics result from the degradation of macroplastics through mechanical, chemical, biochemical, thermal processes, or photodegradation caused by sunlight exposure. This study focuses specifically on macroplastics.

We used the National Geographic-supported Marine Debris Tracker app (2024) to simplify and expedite our data collection. The app can be downloaded and used directly on a mobile device in the field. It offers a straightforward interface for tracking trash and gathering information on inland and marine debris. Each location’s geographic coordinates are recorded along with data on plastic items sorted into predefined categories. Photos taken during debris collection can also be uploaded. Completed sessions are saved and accessible on the app’s main page for future reference. This app is a highly convenient tool for citizen-based litter and marine debris data collection and mapping (Jambeck and Johnson, 2015). We effectively used it to store all relevant site data in a database directly from mobile devices during our field campaign. However, the app does not categorize plastics by chemical type (e.g., PE, PP, PVC, PS), which are more commonly referenced in scientific literature, especially laboratory studies.

The photos in Fig. (4a-c) provide an impression of waste storage in some downstream locations encountered during the field campaign. The photos not only show the state of the plastic items but also their accumulation at these places. It is mainly quiet bays outside of the main flow where plastic waste becomes trapped for a certain time until the next larger flood event.

3.1. Total Occurrence of Macroplastic Items

During field trips to Wadi Hoqain, nearly 2,000 items were found across 11 locations. The items were collected and classified (Fig. 5). The category with the highest count was “Plastic Caps and Lids”, with 661 items, followed by “Plastic Food Wrappers”, which totaled 388. More than 100 items were also found in the categories “Plastic Bottles”, “Plastic Bags”, and “Foam or Plastic Cups or Plates.”

Clearly, the list includes common single-use plastic items typically associated with picnics. Notably, the number of plastic caps exceeded the number of plastic bottles. On average, for each bottle collected, approximately three caps were found.

As expected, plastic items found along their downstream path showed various signs of alteration. Some were broken into pieces, others were deformed, discolored, or even degraded. The photos in Fig. (4) provide a visual impression of these changes. Notably, some items remained almost unaltered, this was especially true for plastic caps.

3.2. Downstream Transport

The sampling campaign started at the main tourist hotspot, the waterfall, and continued downstream for roughly 2 km, with 11 sites chosen for sampling along the route. In the following section, we present the occurrences of the categories starting from stop 1 to stop 11. For each site, an area was marked and measured to relate the number of findings to the size of the area. Table 1 shows the results for plastic bottles and caps.

At nearly all stops, more lids were found than bottles with one exception. At the first stop downstream from the waterfall, the number of caps was nearly six times higher than the number of bottles, whereas at the next stop, the two counts were almost equal. A likely explanation for this discrepancy is that the relatively high water velocity at the first location may carry the lighter, floating bottles downstream to the next pool with lower flow rates, while the heavier caps are more prone to becoming trapped between rocks and pebbles.

Fig. (6) visually presents the counts per area from Table 1. The background of the figure is a satellite image from Google Maps. The sizes of the colored disks indicate the quantity of plastic lids (blue) and bottles (light blue) at the exact locations where they were found.

The coloring of the disks shows no constant pattern. The occurrences of plastic items seem to depend on local conditions in the wadi, similar to what was mentioned above for the extreme differences, as seen in the comparison of findings at stops 1 and 2. However, for both caps and bottles, the figure indicates that the number of found items per unit area decreases downstream, which can be termed a dispersion effect. The numerical values depicted in Fig. (7) clearly demonstrate the declining tendency from stop 1, just downstream from the hotspot, to the final stop 11 downstream.

Figs. (6, 8) depicts the occurrences of plastics categorized as ‘Foam, Cups or Plates and Straws’ at the chosen sampling locations. The highest number per area was 15 items per 100 m² at stop 2, and declined monotonously (with one exception at stop 6), reaching a value one order of magnitude lower at the final stop (Fig. 7).