Arsenic in the Groundwater

When Sarah Jane left Bangladesh to head back home, my husband Chris arrived. Chris’ and my original plan had been to vacation in Bangladesh. But I was not done with the water sampling, so I put Chris to work. He helped me with the last two sampling events in the rainy, cold weather. He is shown below negotiating the slippery bamboo bridge to collect a surface water sample.

Fortunately, the weather warmed up a bit for our last day at the field site. I decided to wear the salwar kameez that Sha’Alam and his wife Nimnee had given me on a previous trip. The outfit was a funny juxtaposition to my trail runner shoes and sun hat.

We finished all of our work by early afternoon, and Sha’Alam and Nimnee hosted a lunch for us before we left. Nimnee is shown below preparing the rice.

The lunch was a nice way to end our time at the field site. Chris and I are now officially in vacation mode. We are planning to visit Cox’s Bazaar, the Bangladeshi beach resort along the Bay of Bengal. The weather is likely too cold for swimming, but that is fine since women are not allowed to wear swim suits and instead have to swim in their salwar kameez.

Although it is winter in Bangladesh, I expected relatively warm temperatures. The weather during the first part of my trip did not disappoint with temperatures hovering near 80 degrees Fahrenheit during the day. However, a cold front has hit Bangladesh and the past few days have been down right chilly. As one can imagine, the cold weather is hard on the locals who are used to the tropical climate and do not have insulated homes.

At first, Sarah Jane and I thought that the Bangladeshis were over reacting a bit to the cold weather. We are from Boston after all and we are used to snowy winters. However, after working a whole day outside in the cold, drizzling weather, we decided it was time to pull out our winter hats.

Working in the cold and rain was not easy. Since I did not bring very many warm clothes to Bangladesh, I was thoroughly chilled by the end of each day. In addition, the ground became very slippery and inches of mud collected on the bottom of our shoes. Collecting water samples became a treacherous process. At one point I slipped off of the bamboo bridge right into the rice field. Fortunately, we had a tent which protected us a bit from the inclement weather.

The rice fields in Bangladesh are irrigated with groundwater that contains arsenic. At my field site, the irrigation water contains 500 ppb arsenic, which is well above the arsenic drinking water limit of 50 ppb. I am trying to track how much of this irrigation arsenic travels through the field back into the aquifer. This effort requires sampling both surface and subsurface water in the rice field over the course of an irrigation cycle (i.e., from one irrigation event to the next irrigation event). During the past six days, Sarah Jane and I have been collecting these water samples. It has been a labor intensive effort with constant work from 7am to 11pm.

Collecting water from the surface of the rice field is fairly straight forward. I installed some bamboo bridges that allow us to reach the sampling location without walking through the field. At each location there are tubes that extend down into the rice field. We pull the surface water up through the tube with a syringe, as Sarah Jane is doing in the picture below.

Collecting water from the subsurface of the rice field is more difficult. The rice field soil is a clayey material from which it is hard to extract water. To collect water, I literally have to pull a vacuum on the water sampling devices that I have installed at different depths under the field. Even with a strong vacuum sucking on the clay, it can take 12 hours to collect 100ml of water. I am shown below, along with the two Bangladeshi men that I hire to help me, hooking up the vacuum pump (the square box) to one of my collection devices (the glass jars with the black stopper lids).

The problem with the slow collection rate for the subsurface water is that the water has plenty of time to get exposed to oxygen in the atmosphere. The atmospheric oxygen can cause chemical reactions to occur which would not normally occur in the subsurface where there is little to no oxygen. These reactions can involve arsenic and therefore impact our results. I try to limit the water’s exposure to the atmosphere by collecting it directly into bags that are made with an oxygen-barrier material. Furthermore, I purge the bags with argon before using them. The purging process ensures that there is no oxygen inside the bags. Sarah Jane is shown below purging the bags.

Since we were collecting subsurface water every day and night during the irrigation cycle, we needed to have the argon tank at the field site so we could purge the bags in the morning before the day-time collection period and then purge more bags in the evening before the night-time collection period. As one can imagine, it is not easy to get a compressed gas cylinder out to a rural village. My two Bangladeshi helpers (Sha’Alam and Rassil) had to carry it the quarter mile distance from the car to the field site. For those who do not know, a compressed gas cylinder can be very dangerous. If the top of the tank gets knocked hard, all the compressed gas will escape at once and the heavy metal tank will go flying like a rocket. Field work in Bangladesh is never short on excitement.

After my slow progress and a few equipment failures (below is a picture of our currently working generator a few days ago), I was afraid that I would leave Bangladesh with many uncompleted tasks.

But a fellow graduate student, Sarah Jane, arrived two days ago to help me and together we are making good progress. We have been collecting water samples from the rice field and the aquifer so I can determine their chemical composition. Yesterday, Sarah Jane calibrated the water quality probes while I got the infrastructure set up to collect samples. It was a very efficient arrangement. She is shown below during the aquifer sampling standing next to the probes that she calibrated.

Our only hurdle now is the fog that blankets the countryside in the morning, which makes getting an early start to the day a bit dangerous. In the fog, the visibility on the highway in only a few feet. However, this does not seem to stop our driver or the other drivers on the road from driving extremely fast. Below is a picture of the village at the field site in the morning fog.

My first field objective was to figure out the flow patterns of water through the rice field and to determine exactly how much rice-field water reaches the arsenic contaminated aquifer every year. To ascertain flow patterns, I installed water-pressure sensors at different depths in the field. Water flows from high pressure and elevation to low pressure and elevation. Knowing the relative elevation of each sensor and the pressure at their locations allows me track water movement. The picture below shows one of these sensors (the long blue rod) and a group of us debating the best installation method.

I hooked all of the sensors up to a datalogger, which is essentially a simple computer, and programmed the logger to take a measurement every hour. Therefore, even when I am back in the USA, I am collecting data (too bad my adviser does not count this as working).

The one problem with using electronic equipment at this field site is the monsoon season. During the summer, the site can be inundated with up to 15 feet of water! The rice fields are transformed into endless expanses of water. The only way to keep the datalogger dry and the sensor system working, was to house the logger above the monsoon flood level. Being an MIT student, I built a 20-foot tall tower.

The wires from the sensors run under the rice field and up the middle of the tower. Inside the cage of the tower is the datalogger and the battery that powers the sensor system. To download data off of the logger I have to climb the tower.

All of this work showed that a majority of the water in the rice field flows down the bunds, which are the raised boundaries around the edge of the rice field. Water flows down the bunds because the farmers do not plow through the bunds when they plow the rest of the field. Therefore, the soil under the bunds is less consolidated than in the rest of the rice field, and cracks under the bunds are never sealed. These factors make it easier for water to flow through the bunds than elsewhere. Bund flow is interesting because it is relatively rapid, which has implications for arsenic chemistry. In addition, if the farmers were to plow through the bunds, they could potentially save a lot of water.

After five field visits, I should have a good sense of the time it will take me to complete certain tasks. However, I am currently four days behind the schedule that I set for myself! I have been waking up at 4am to work in the hotel room before heading to the field, and then working all day at the field. By the time I get back to the hotel, I am exhausted. It takes all of my energy to shower and eat dinner before crashing into bed.

I am not sure where my time-estimation went awry. When I plan my field work, I include excess time for tasks since things always do take longer than expected. But this trip things are moving exceptionally slowly.

The shallow aquifer in Bangladesh, which provides drinking water for millions and irrigation water for numerable rice fields, is severely contaminated with naturally occurring arsenic. The arsenic exposure is poisoning millions of Bangladeshis and causing thousands to die each year from arsenic-induced cancers. My research group is trying to determine the causes of the high concentrations of arsenic in groundwater.

Rice fields cover approximately 65% of the land area in our study area and water-balance calculations show that they contribute about half of the water that recharges the arsenic-contaminated aquifer every year. When I began my research, it was not clear what role rice fields play in the contamination problem. Rice fields can develop the chemical conditions needed to move arsenic off of the soil and into the water. (All soil, everywhere in the world, contains arsenic. Usually the arsenic remains associated with the soil and does not pose a health threat, but under certain chemical conditions it will dissolve into the surrounding water.) Thus, it is possible that the fields are contributing to arsenic to the shallow aquifer. However, the fields are irrigated with arsenic rich groundwater, which means a significant amount of arsenic is removed from the aquifer and placed onto the fields every year. Therefore, it is also possible that the rice fields act as a net sink for arsenic from the aquifer (i.e., more arsenic is removed from the aquifer then enters the aquifer because of the rice fields).

Figuring out the role of rice fields requires an investigation of both the hydrology and chemistry of a field. I need to determine flow patterns through a rice field and then investigate water chemistry along known flow paths. I will provide more information about my research methods and results in subsequent posts.

Bangladesh has gotten more expensive since the last time I was here. The cost of everything (labor, food, lodging) has increased by more than 30%. I think the main cause for this increase is the cyclone that hit the country this November. My understanding is that the storm ruined a lot of current crops as well as crop land. So now there is a shortage of rice in the country. Everyone is telling me that rice used to cost ~15 taka/kg and now it cost ~35-50 taka/kg (the price fluctuates as the government tries to manage the food shortage). As you can imagine, this increase in the cost of the staple food is difficult on everyone living here. I am not an economist, but I think the increased cost of rice is driving up the prices of everything else.

Rice fields have fascinated me ever since my first trip to Asia when I was in high school. It amazes me how they dominate the landscape.

Now I get to actually work in a rice field. Working in a rice field is not an easy task. When you step into the field, you sink down ~6 inches into unconsolidated soil (aka, muck). If your boots are not securely attached to your feet, then you leave them behind when you try to walk. The difficulty of walking is increased by the fact that you should not step directly on the rice plants, but need to step between them. This requirement has caused me to lose my balance and fall face first into the field.

Most of my work involves burying sensors and water samplers in the field. The difficulty of digging and backfilling holes for these research instruments cannot be underestimated. The hole immediately fills with water, and if you do not mark its location, you can easily lose track of where it is. However, this is not as bad as dropping a tool, like a screwdriver or a wrench, into the rice field. When this happens, the tool is likely never seen again. My rice field has eaten a wrench or two. It goes without saying that all of the equipment and myself gets covered in mud.

Even though I get covered in mud almost everyday, the villagers at the field site still seem surprised at how dirty I get. They politely point out all of the mud spots that I would not be able to see on myself. I usually don’t mind being mud-covered. But it can be a bit embarrassing when, at the end of the day, I have to stop by BUET (Bangladesh University of Engineering and Technology). I know the students and professors stare at me in wonder: the foreign girl covered in mud.

After traveling to the other side of the globe and suffering through a few sleepless nights due to jet lag, I am now settled into the fieldwork routine. I wake up at 6 a.m., eat breakfast at 6:30 a.m., and drive to the field site at 7 a.m. (assuming the driver arrives on time, which can be a horribly wrong assumption sometimes). I then toil away the day at the site and try to leave before it gets dark. My field site is located about 20km south of Dhaka and the drive takes anywhere from 45 minutes to 2+ hours, depending on traffic. The drive in the morning is fairly pleasant because there is never traffic. But the drive home in the evening is dependably bad because the streets of Dhaka are always very congested. I’ve never seen traffic this bad in any other country that I have visited. I think Bangladesh must top the world’s worst places to drive.

This is my sixth trip to Bangladesh, so I have developed some nice relationships with the villagers at the site. I am especially close to the men that I hire to help me. One of the men I usually hire, Shojib, has left to work in Dubai. He left for Dubai over the summer and plans to be there for three years. I can’t imagine leaving my family for that long. Sha’Alam is the other man that I usually hire. Unfortunately, his house was blown down by the recent cyclone that hit Bangladesh. From what I can see, his house seems to be the only one that sustained any permanent damage from the storm. He and his family are currently working to rebuild. Fortunately for me, most of my field equipment survived the storm.

(To read a summary of my research visit the CEE Department web site)