Long-Term Environmental Impacts of Pesticide and Herbicide Use in Panama Canal Zone

The opening of the Panama Canal in 1913 transformed ocean-shipping and the availability of internationally-traded goods, shortened travel time between the Pacific and Atlantic oceans, increased ship tonnage, and sparked the growth of port authorities on both the Atlantic and Pacific coasts of the Panama Canal. Historically, the United States was number one and China was number two in tons of cargo that pass through the canal annually in the high stakes game of import and export markets. Prior to the construction of the Panama Canal, the most efficient way to cross the 82-kilometer isthmus, between the Port of Panama City on the Pacific and the Port of Colon on the Atlantic, was by mule trails through tropical forests and river transportation. Since the construction of the Panama Canal through tropical forests in the 1910s, pesticides have been essential for managing mosquitoes as well as controlling wetland vegetation that blocked lakes, rivers and the canal. The primary objective of this research study is to document the long-term environmental impacts of pesticide and herbicide use in the Panama Canal Zone. Many of these chemicals, including 2,4,-D, 2,4,5-T and DDT, have a long half-life under water and some, like arsenic (As), have no half-life. Pesticides and chemicals flowed into Lake Gatun via surface runoff either in solution or attached to the sediment during the rainy season. The by-product 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is an unanticipated contaminant created during the manufacture of the herbicide 2,4,5-T. TCDD can bio-accumulate in fish and birds and enter into the human food supply. The extent of the current chemical and pesticide contamination on former U.S. military base grounds and in Lake Gatun is unknown. Systematic soil sampling of current and former military bases, chemical disposal sites and Lake Gatun or the Panama Canal sediments is needed to determine if mitigation is necessary. How to cite this paper: Olson, K.R. and Tornoe, D. (2021) Long-Term Environmental Impacts of Pesticide and Herbicide Use in Panama Canal Zone. Open Journal of Soil Science, 11, 403-434. https://doi.org/10.4236/ojss.2021.119021 Received: August 20, 2021 Accepted: September 4, 2021 Published: September 7, 2021 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
Panama, a tropical nation is located on the Isthmus of Panama with 1,600 islands along its Atlantic and Pacific Ocean coasts ( Figure 1 This cosmopolitan city of skyscrapers has become the financial and commercial center of Central America. The economic progress has been hampered by environmental problems and political turmoil. Panama and its capital enjoy a lively mix of cultural influences which include artwork, cuisine, literature and music [3]. The primary objective of this research study is to document the long-term, more than 100 years, environmental impacts of historic pesticide and herbicide use on the soils and water of the Panama Canal Zone and review the need for sampling of soils of former military bases and sediments in Lake Gatun and Panama Canal Zone for DDT, dioxin TCDD and As to determine if mitigation is necessary.                   Ocean [3]. Only 30 km of the Chepo River and 60 km of the Tuira River are navigable during the rainy season. Water in the Panama Canal is released from the central highlands rain-fed Lake Gatun ( Figure 19) and ( Figure 20) and Alyuela lakes.

Location of Site
The soils are clay rich and either brownish or reddish in color [4]. Soil productivity varies with many crops requiring fertilizer to be grown successfully.
Shifting subsistence agriculture, called roza, is practiced on the less fertile soils.
Under "shifting cultivation", small plots are cleared and cropped for a few years and then abandoned to naturally restore the soil's natural fertility. Alluvial soils formed in stream deposited sediments are in the lower parts of river valleys.
Soils along coastal mangrove swamps are productive [4] while the soils developed from volcanic ash are exceptionally fertile. Unfortunately, artificial Lake Gatun, when constructed, covered many of the most productive alluvial soils in Panama.      The construction of the Panama Canal had a great influence on the culture and economy of Panama City. Minorities from all over the world flocked to Panama for work and passed through the canal because it was the shortest route to the gold fields of California and Alaska. Areas of Panama have quicksand where railroad cars have been known to disappear, making construction activity difficult. The Commissary would stock opium for the Chinese workers, which eventually became a problem. Apparently the opium helped them with the unpleasant work and surroundings. When the company commissary stopped stocking opium many workers committed suicide.
Spanish is the official language of Panama. More than 90% of the population speak Spanish and 10% speak American Indian languages. Roman Catholicism is the religion of 75% of the Panamanians, but the number of Protestants grew rapidly in the late 20 th century. Today the most densely settled part of Panama is Panama City [1]. The population in 1911 was 336,000 and by the 1990s had increased to 2,688,000. Panamanians live predominantly in urban areas with only 30% living in isolated dwellings and hamlets with many engaged in subsistence agriculture.

Economy
The service sector generates 75% of Panama's gross domestic product (GDP). Services are related to canal traffic ( Figure 22), public administration, mineral extraction ( Figure 23) offshore banking and other services. Fishing and agriculture accounted for less than 10% of the GDP prior to 1990s [1]. Prior to the 1990s, U.S. military forces stationed in Panama supported 5% of GDP. After U.S. troop withdrawal, the Panamanian government lost millions of dollars. The Panamanian government has re-developed ( Figure 24) the previously U.S. controlled properties in the Panama Canal Zone and prime real estate in Panama City. The government repaired the road and rail system and has promoted ecotourism. China, Latin America, Middle East and the U.S. have all invested in

Agriculture, Forestry, and Fishing
Land tenure was enacted in 1962. Now modern commercial agricultural enterprise are adjacent to subsistence and seminomadic agriculture.

Resources and Power
Salt, limestone and clay are the leading mineral products ( Figure 23). On a small scale gold, manganese and ferrous sand are also being mined. There are significant deposits of copper, phosphates, bauxite, and coal that have been minimally exploited [3].

Pesticide and Herbicide Use in the Panama Canal Zone
During the canal construction phase, the workers were at great risk of dying from malaria and yellow fever. In an attempt to reduce the spread of these diseases, window screens were used on structures and the swamps and wetlands were drained to control the mosquito populations. The Panama Canal was completed and opened in 1913 [3]. The canal channel went through the newly created Lake Gatun, which was periodically covered with hyacinths [5] [6] [7] and other floating plants. An early attempt to eliminate the floating plants was to purchase commercially available white arsenic and mix it with soda bicarbonate (soda) and water [8]. This mixture was then sprayed on floating plants including The U.S. Army technical manual [9] and the U.S. Army Medical Department Center and School subcourse [10] for groundskeepers were used to provide guidance on how to eradicate specific weeds, such as kudzu, and insects on the military base grounds (Figures 25-30). The military properties, housing units, and connecting roads were sprayed daily using truck-mounted sprayers apparently filled with DDT. In the 1940s and 1950s termite stake studies [7], with      stakes dipped in pesticides and other chemicals including lead arsenate, sodium arsenite and DDT were conducted by the USDA, Forest Service on Barro Colorado Island where the Smithsonian headquarter buildings are located [7]. In 1963 the Curundu Jungle Test site at Fort Clayton on the Pacific side was used to determine which pesticide and chemically treated stakes would not be eaten by the termites. The commercially available herbicide, 2,4-D was used in Lake Gatun since 1948, which was the feeder lake water source for the Panama Canal.

Yellow Fever
The Istmus of Panama was an ideal environment for mosquitoes [11] [12]. Yellow fever was the most feared of the scourges afflicting humans in Panama. Many workers thought the "miasmas" or heavy, bad-smelling vapors that rose out of the wetlands and swamps were the cause of diseases. The "miasma theory" of disease causality persisted from the 13 th century through the 19 th century, even with the discovery that mosquitoes (Aedes aegypti) carried the disease which hindered the Gorgas sanitation plan in 1904. The initial symptoms of yellow fever were a headache, fever and back pain, followed by prostration, bleeding and decreased urine production. Finally, delirium and black vomit brought death ( Figure 31) within a few days. During the July 10, 1904 to November 22, 1905 time period, there were 246 cases. Most patients were treated at the Anoco Hospital ( Figure 32) in Anoco, Panama [8].

Malaria
The successful construction of the Panama Canal required the control of malaria. Sir Robert Ross, a British Doctor, discovered in 1907 that malaria was transmitted by mosquitoes. In Cuba, regulations were put into effect in 1899 by  The same concept was applied a few years later in Panama [8]. In June 30, 1904 the Sanitary Department was created and headed by Colonel Gorgas. He applied a military style campaign to eradicate mosquitoes, which, while costly, greatly reduced the breeding of mosquitoes and yellow fever and malaria transmission [8].

White Arsenic
The white arsenic (As) used between 1913 and 1932 to kill the floating plants in both Lake Gatun and the Panama Canal is an extremely toxic soluble white solid made by burning arsenic (As 2 O 3 ). Arsenious oxides, commonly known as white arsenic, are obtained as a by-product from the smelting of lead, copper, and certain other metals by roasting of sulfide ores and arsenopyrite. Sweden was the United States primary source of white arsenic in the 1960s. The inorganic forms, including arsenate/arsenite (found in soil, water and food) and arsenic trioxide, are human health concerns [3].

Barro Colorado Stake Termite Stake Study
The USDA, Forest Service [7] conducted chemical and pesticide dip stake stu-

Malathion
Malathion which acts as acetyl cholinesterase inhibitor is an organophosphate insecticide that affects the nervous system and increases the risk of Parkinson's disease [11]. Malathion was registered for use as an insecticide in the United  [14] before they were sent to Howard Air Force base in Panama [14]. These aircraft were used to spray malathion on the Panama Canal Zone from 1970 to 1973.  2,4-D kills both terrestrial and aquatic broadleaf weeds [17]. It was and is still used to control aquatic weeds in the Panama Canal and Lake Gatun which interfered with boating and shipping. In other waterways it was used to unclog irrigation canals and hydroelectric equipment. Some ester forms of 2,4-D are Under most environmental conditions 2,4-D esters and amine salts are not persistent [15]. The herbicide 2,4-D is broken down by microbes in soil in a process involving hydroxylation. A number of 2,4-D degrading bacteria have been isolated and characterized from a variety of environmental habitats. Breakdown is pH dependent, but is highly mobile and leaches as a result of low affinity in mineral soils and sediment.

2,4-Dichlorophenoxyacetic Acid
In an aerobic mineral soil the degradation of 2,4-D is rapid (half-life of 6.2 days) [18]; however it has a 15-day half-life in anaerobic mineral soils. In anaerobic aquatic environment, 2,4-D was moderately persistent to persistent (half-life of 41 to 333 days) [18]. The herbicide 2,4-D has been detected in streams and shallow groundwater but at low concentrations in United States rural and urban areas.
No data are available for the Panama Canal Zone.

The Herbicide 2,4-5-Trichlorophenoxyacetic Acid
A chlorophenoxyacetic acid herbicide 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) is a synthetic auxin created to defoliate broad-leafed plants. It was developed in the late 1940s and was slowly phased out in United States in the late 1970s due to toxicity concerns and not manufactured after 1985 [19]. The herbicide 2,4,5-T itself is toxic.
An intake rated of 10 mg/kg/day 2,4,5-T can cause adverse alterations in organisms.
The half-life of 2,4,5-T is similar to 2,4-D. The degradation of 2,4-D and 2,4,5-T are rapid (half-life of 6.2 days) in an aerobic mineral soil [18] but have a 15-day half-life in anaerobic mineral soils [18]. The herbicides are moderately persistent to persistent (half-life of 41 to 333 days) in anaerobic aquatic environments.
TCDD has a very long half-life and does not degrade easily. Each batch of commercially available 2,4,5-T herbicide has an average concentration of about 2 to 3 ppm of TCDD but can vary between 0.05 ppm to 50 ppm [19]. TCDD is an endocrine disrupter and can cause a variety of chloracne, cancers and developmental and reproductive effects.
TCDD is not water soluble and can adhere to leaf surfaces, organic material, fine soil particles and sediments which can be carried downstream by runoff water flow into wetlands, ponds and lakes such as Lake Gatun. Dioxin TCDD can bio-accumulate in aquatic species and become bio-magnified throughout the food chain via mollusks, fowl and fish eaten by humans and other animals [20].
The Lake Gatun and Panama Canal sediment should be checked for concen-Open Journal of Soil Science trations of TCDD which is probably bound to organic materials and soil particles within the lake, wetlands, waterways and ponds. TCDD could be diluted in Lake Gatun depending on the distance from shore. TCDD can be eliminated from the surface of Lake Gatun waters by photo degradation. The half-life of TCDD in the clear waters of Lake Gatun is based on high intensity of ultraviolet radiation hitting the lake waters. The largest fraction of TCDD in most waters binds to detritus, algae, organic materials and mineral matter. These TCDD particles do not remain in the water column but attach to and are deposited with the sediment. By measuring particle concentration in water and their rate of settling the rate of loss can be calculated. Very little TCDD is dissolved in water due to its hydrophobic nature. TCDD in the sediment particles can be returned to the water when sediment is re-suspended [21].
Sediment re-suspension occurs in Panamanian lake waters which are shallow and easily churned by wave and wind action [22]. TCDD is ultimately eliminated from the aquatic system via microbial degradation within sediments or when TCDD particles found in sediments are buried deeply in waterways or lakes [22].
Degradation by anaerobic microorganisms in sediments or permanently saturated soils can occur over time periods of 10, 20, 50 years to centuries [18].
TCDD in the food chain affects both Panamanian capacity to derive livelihoods from fish and other aquatic plants and animals and their own health.
The half-life of dioxin varies depending on site conditions. In surface soil the half-life is between 1 and 3 years when fully exposed to sunlight. In an aerobic soil the half-life is between 11 and 15 years and estimated to be 20 to 50 years or more when buried in the tropical anaerobic soil. In waterways and lake sediments the half-life can be more than 100 years [18]. Although TCDD is not water soluble and can adhere to leaf surfaces, organic materials, fine soil particles, and sediments that can be carried by runoff into Open Journal of Soil Science downstream waters and deposited in wetlands, ponds, and lakes [24] [25]. In this way, TCDD can bio-accumulate in aquatic species and can become biomagnified throughout the food chain via mollusks, fowl and fish eaten by humans and other animals [20] [24].

Transport and Fate of Agent Orange and Commercial Herbicides 2,4-D and 2,4,5-T Containing Contaminant Dioxin TCDD
A recently released report of the United States Government Accountability Office (GAO) focused on the actions needed to improve the accuracy and communication of information regarding storage and testing locations of Agent Orange outside of Vietnam [14] including Kelly Air Force Base in Texas [26].

U.S. Federal Government and Military Use of Commercial Herbicides with 2,4,5-T with Dioxin TCDD Contaminant in Panama Canal Zone
The U.S. government and military have for more than 60 years maintained those tactical herbicides such as Agent Orange and Agent Blue were never "off loaded in the Panama Canal Zone nor aerially sprayed on the Tropical Forests of the Panama Canal Zone" [28]. This statement has over the years been challenged by many Panama Vietnam Era Veterans who claimed that they handled and/or were exposed to toxic chemical sprays [14]. Panama veterans who were exposed to commercial herbicides including 2,4,5-T containing unknown amount of dioxin TCDD, have filed for VA benefits but have been repeatedly denied benefits [12] since the tactical herbicides were not officially offloaded and/or applied to the Panama Canal Zone landscape [27]. However, commercial herbicides con-Open Journal of Soil Science  [27], that tactical herbicides were never used in the Panama Canal Zone. However, DOD was not really claiming that "unrestricted commercial herbicides" were never requisitioned, offloaded, handled and applied by military personnel to the military base grounds and perimeter fences [9] [10] [12] at the direction of the Base Civil Engineer Commanders. It is also assumed that any aerial spraying done by aircraft only involved commercially available herbicides including 2,4-D and 2,4,5-T containing dioxin TCDD and arsenic [11]. However, three C-123 aircraft that were used to spray malathion in the Panama Canal Zone may have been previously contaminated with tactical herbicides in other theaters of operation including South Vietnam. After President Nixon stopped Agent Orange spraying in 1970, three C-123 aircraft previously used to spray tactical herbicides in Vietnam [28] and contaminated with tactical herbicides (including Agent Orange, Agent White and Agent Blue) residues in their spray tanks, were relocated via a transport ship in 1970 to Howard Air Force base in Panama. These TCDD and As contaminated C-123 planes were used from 1970 to 1973 to spray malathion in the Panama Canal Zone [28]. This may have resulted in trace amounts of tactical herbicide Agent Orange (TCDD) and Agent Blue (As) [29] being added to the soils and water of the Panama Canal Zone.
A GAO review of DOD and VA documents [30] identified multiple examples of incomplete and inaccurate information on the DOD's list of tactical herbicide test and storage sites such as Kelly Air Force Base in Texas. GAO obtained command histories and original DOD reports which provided operational details about the procurement, distribution, use, and disposition of Agent Orange [20] and its components, 2,4-D and 2,4,5-T containing unknown amount of dioxin TCDD. GAO concluded that there was extensive documentation on the herbicide management program at Kelly Air Force Base for the American Vietnam War time period and more specifically years 1966-1973. According to an Air Force Logistics Command's Office of History monograph, the command directly responsible for managing Agent Orange was the Directorate of Aerospace Fuels at the San Antonio Air Material Area located at Kelly Air Force base [14]. During the Vietnam War Kelly Air Force Base was also a subcomponent of the U.S. Air Force Logistics Command.
GAO documentation shows that quantities of the two components of the tactical herbicide Agent Orange were stored at Kelly Air Force Base in Texas in 1972. There were 38,940 gallons of 2,4,5-T containing TCDD and 106,260 gallons of 2,4-D stored on the base [14]. The uneven quantities of these two herbicides suggest that not all the tactical herbicides in storage were in the pre-mixed to form of Agent Orange, Herbicide Orange or Butyl Ester. Perhaps Agent Open Journal of Soil Science White a tactical herbicide used when Agent Orange was in short supply, which contains only 2,4-D but not 2,4,5-T with TCDD was also in Kelly Air Force base storage [14]. After the American Vietnam War ended in 1973 the tactical herbicide Agent Orange with 2,4-D and 2,4,5-T containing an unknown amount of dioxin TCDD herbicide components in the stock pile was apparently transferred to the United States Department of Agriculture (USDA) for brush control which could have been one of the sources of the herbicides used by the USDA, Forest Service in the Western United States in the 1970s to control the brush and broadleaf weeds after clear cutting. This was the subject of a 2021 PBS documentary Independent Lens on "People vs. Agent Orange" [31] and a Vietnam Veteran News podcast 2086 by Mack Payne [32]. In order for the tactical herbicides stored at Kelly Air Force base to be transferred to the USDA for agency use in the Western United States it appears that the tactical herbicides would have had to be reclassified from formulated "Military Specifications" [33] to formulated "Federal Specifications" before being approved for use and applied by the USDA, Forest Service, timber company and chemical company partners in the Western United States to prevent shrub and broadleaf weed regrowth competition with trees on clear cut public forest lands.

Fate of Tactical Herbicides Stored at Kelly Air Force Base at the End of the American Vietnam War?
GAO records also show that approximately 173,910 gallons of the tactical herbicide Agent Blue [14] containing cacodylic acid (As) was also stored at  Figure 33 and Figure 34). This would have been in

Conclusions
The Panama Canal linked the Pacific and Atlantic coast ecosystems and destroyed a natural barrier, the Isthmus of Panama, by connecting of the two oceans, creating Lake Gatun on the Chagres River, covering up fertile agricultural lands and swamps. During the Panama Canal construction phase, many workers died from tropical diseases including malaria and yellow fever. To reduce the mosquito population, malathion was sprayed at the canal construction site since the 1960s. Malathion has a relatively short half-life and does not appear to degrade into other environmentally harmful by-products. After the damming of the Chagres River to create the Lake Gatun for the Panama Canal, floating plants, including hyacinth started to clog the flow of the water through Lake Gatun. A mixture of white arsenic, soda and water was sprayed from 1913 to 1930s from boats on the floating vegetation. The arsenic, which has no half-life and is water soluble, was deposited in Lake Gatun waters and sediments. When the three contaminated C-123 aircrafts were shipped from South Vietnam to Howard Air Force Base in 1970, with the tactical herbicides residues were still in the spray tanks, containing high levels of TCDD and As. These contaminated C-123 aircraft and spray tanks were then used to aerially spray malathion on the Panama Canal Zone landscape in an attempt to reduce the number of malaria cases. The tactical herbicide residues in the spray tanks would have mixed with malathion and then sprayed on the Panama Canal Zone soils and water. It is true that only trace amounts of As and dioxin TCDD would have ended up on the Panama Canal Zone from the spraying of malathion. Since TCDD has a long-half life under anaerobic conditions in soil and water and As has no half-life the trace amounts of these toxins would still be present in the Panama Canal Zone soils and water.
The C-123 flight crews, maintenance specialists, and mechanics would have been exposed to the tactical herbicide residues containing TCDD and As. Some of these U.S. Air Force, Panamanian Vietnam Era veterans have already filed VA benefit claims as a result of using and serving these contaminated aircraft [12] [29]. The VA Office of Compensation has continued to support the DOD position that "no tactical herbicides were ever offloaded or sprayed in the Panama Canal Zone". However VA was willing to make a one-time exception for the flight crews and the maintenance mechanics assigned to these three contami- Department and Center and School sub course [10] gave very specific instructions to the grounds crews on how to deal with each type of weed. For example, kudzu was to be killed by spraying a commercially available herbicide, 2,4,5-T.
During the post-Vietnam War years the expanded use of this herbicide containing unknown amounts of TCDD increased dioxin TCDD levels in the environment. Pesticides and chemicals flowed into Lake Gatun via surface runoff either in solution or attached to the sediment. These pesticides including 2,4,5-T containing TCDD, can be bio-accumulated in fish and birds and feed and enter into the human food supply [13]. The extent of the current chemical and pesticide contamination on former Panama Canal Zone U.S. military base grounds and in Lake Gatun and the Panama Canal channel is unknown. Systematic soil sampling of former military bases, chemical disposal sites, and sediment sampling of the Lake Gatun or the Panama Canal sediments is needed to determine if mitigation is still required.