## 1 Introduction  
The Shat AI-Arab river, the primary freshwater source in a relatively arid region [1], delivers lifeline advantages for millions of people in its basin [2]. The river's basin encompasses the major oil and gas fields in Iraq and Iran. The oil industry is the main artery for the federal budget in these two countries. The river provides water for domestic, industrial, agricultural, transportation, natural ecosystems, and recreational purposes [3]. Moreover, the river is the main freshwater source for the gulf and is essential to maintaining the marine ecosystems along the gulf's northeastern coasts [4, 5]. However, the oil industry, which has grown sharply in the last two decades, accompanied by population growth, has resulted in rigorous environmental impairment that not only threatened the environment but also made it impossible to maintain the consequent economic growth [3, 6]. Industrialand residential waste represent major sources of pollution in the basin. Furthermore, agricultural practices are another source of contamination in the basin [7; 8]. Usually dumped into the river, contaminated water has several long-term effects on the environment and human health [9]. When excessive nutrients, primarily nitrogen and phosphorus, enter water bodies, they promote excessive algae growth, known as eutrophication [10]. Cultivation significantly affects the stream's nutrient concentrations [11]. N and P concentrations in cultivated land are generally high due to fertilization and agricultural management practices such as tillage and irrigation, which promote eutrophication of the water [12; 13]. Urbanization is commonly linked to alterations in the physical and hydrogeological properties of basins as well as the formation of new nutrient sources. Sewage discharge, garden fertilizers, and domestic animal wastes in urbanized areas are all associated with high N and P loading and, as a result, water eutrophication [14; 15; 16]. Eutrophication has been attributed to increased blooms of algal organisms, water turbidity, oxygen depletion, the prevalence of certain species, and biodiversity loss [17; 18]. For example, oxygen deficiency can mobilize elements such as heavy metals previously precipitated or bound to sediments [19]. Such metals adversely affect ecosystem health and water quality, e.g., [9; 20; 21]. Maintaining clean water is vital for habitat preservation, agricultural yield, and public health. Several research works on the evaluation of pollution status in the Shatt Al-Arab river have provided good knowledge of river pollution in terms of sources, processes, and management techniques [22; 23; 24; 25]. For instance, [PERSON] et al. [5] concluded that Shatt Al-Arab's coastal pollution from upstream urban, industrial, and agricultural expansions has the potential to create considerable ecological and socioeconomic implications. [PERSON] et al. [26] collected 14 samples along the middle part of the Shatt Al-Arab river, starting from Shilha town to Abu Al-Khasib city, and reported high N and P loads. Likewise, [PERSON] [27] collected water samples monthly from October 2011 to October 2012 from five locations along the Shatt Al-Arab main course. The author detected N and P in high concentrations, demonstrating organic pollution in the river. Similarly, [PERSON] et al. [28] reported N and P contamination in the river and attributed it to agriculture runoff and untreated sewage disposal. [PERSON] et al. [29] collected 210 samples from 53 locations quarterly during 2018-2019 along the Karun river and its branches. They reported N and P concentration ranges of 6.79 to 10.91 and 0.49 to 1.78 mg L\({}^{-1}\), respectively. Such concentrations exceed the acceptable levels set by the EPA (2-6 and 0.1 mg L\({}^{-1}\) for N and P, respectively) [29; 30]. These studies, however, were restricted to a certain reach of the river. Thus, it is essential to comprehensively identify the spatial distribution features of the nutrient contamination in the entire Shatt Al-Arab basin.