3.2.1. Pathogens

Often because of its nutrient content and accessibility, untreated wastewater from municipal and domestic sources containing excessive levels of pathogens is often directly used for irrigation in developing countries [6]. Untreated wastewater generally carries a high pathogen load compared to other irrigation water sources. Risks from pathogen (bacteria, viruses, or protozoan or larger organisms) contamination to irrigation water quality will continue to be a topic of primary concern [115–117], and it is impossible to adequately address this topic in a few paragraphs. Pathogenic microorganisms in irrigation water likely pose the greatest acute risk to human health and will continue to be a concern especially in freshly-eaten produce. Pathogens are biological organisms that may influence modern-day irrigation water quality. Pathogen contamination is generally related to surface water sources, but groundwater may also be under threat if it is recharged with wastewater sources [116]. The complexity of reproducibly measuring microbiological contamination of irrigation water has made monitoring difficult. Several different types of pathogens have been detected in diverse irrigation water sources including bacteria (e.g., *Salmonella* and *Escherichia coli*), protozoa (e.g., *Cryptosporidium* and *Giardia*), as well as viruses (e.g., noroviruses)) [118,119]. Irrigation of food crops with surface water clearly has the highest potential for contaminating freshly eaten produce, and this topic has had the greatest research and regulatory effort in recent years.

There have been quite a few comprehensive reviews emphasizing irrigation water as a source of pathogenic microorganisms in fresh produce [120–123]. Between 1973 and 2012, the Centers for Disease Control and Prevention reported 606 leafy-vegetable associated pathogenic outbreaks (norovirus (55% of outbreaks), Shiga toxin-producing *Escherichia coli* (STEC) (18%), and *Salmonella* (11%)), with 20,003 associated illness and 19 deaths [124]. From 2013 to 2017, the number of outbreaks (mainly from norovirus (32%) STEC (23%), and *Salmonella* (32%)) associated with leafy greens and vegetables decreased to 21, with 699 illness and five deaths [125]. In 2018, 272 infections were reported from two outbreaks (*E. Coli*) associated with romaine lettuce resulting in five deaths [126,127], and another multi-state outbreak was linked to parasite *Cyclospora*, which reported 511 cases of infection [128]. However, a 2014 risk-based review conducted in California suggests that recycled water quality criteria, along with proper agricultural management practices do not lead to increased public health risk [129]. In the US, the Center for Produce Safety has published information on the factors that affect the microbiological safety of agricultural water [130]. The Foodborne Disease Outbreak Surveillance System (FDOSS) has an online tool, the National Outbreak Report System (NORS), which keeps track of outbreaks in the United States. Reports of pathogen contamination from inadequately treated wastewater have also been documented in developing countries [8]. The occurrence of pathogens in water used to irrigate food crops is considered a severe problem affecting human health both in both developing and even in developed countries [131]. Groundwater sources are generally considered less vulnerable to contamination by pathogenic microorganisms, while surface water and wastewater have a much higher potential for contamination. Farmers utilizing surface water for food crops, which are consumed raw should follow proper mitigation strategies to control contamination [132]. The method utilized for irrigation has a substantive role in pathogenic contamination of crops. For example, subsurface drip may have the lowest risk as the water is generally applied at the root zone, unlike other methods (e.g., sprinkler irrigation) where the edible portions of crops can come in contact with contaminated water [133]. New and more intensive monitoring approaches and potential disinfection and treatment techniques for surface water used to irrigate food crops are needed to improve food safety [8].

### 3.2.2. Antibiotic Resistance

The World Health Organization (WHO) has listed antibiotic resistance among today's biggest threats for global health, food safety, and development, as this threatens the ability to treat common infectious diseases [134]. Antibiotic resistome is defined as the sum of all genes directly or indirectly contributing to antibiotic resistance both in the clinics and the environment [135]. Aquatic ecosystems are regarded as a primary reservoir of antibiotic-resistant bacteria (ARB) [136]. The presence of ARB and their resistance determinants in surface water sources have been well documented and is generally linked to nearby wastewater treatment plant effluent [137–141]. Wastewater treatment plants enrich ARB and their resistance determinants as it favors exchange of antibiotic resistance genes (ARG) among bacteria and selection of resistant strains [142]. In a recent study, it was found that multidrug-resistant (MDR) bacteria were found to be more prevalent in surface waters than in treated wastewater [143].

Irrigation water is one of the major sources for contamination of fresh produce with antibiotic resistance bacteria [58,144,145]. Similar to pathogens, the incidence of ARB contamination is higher when using overhead sprinklers as water can directly come in contact with fresh produce. When fresh produce is consumed raw it can act as an ideal vector for exposure. The diversity of ARB present in fresh produce is significant and can have a severe impact on human health.
