Jackson's boss, EPA administrator Scott Pruitt, is the target of more than 10 open investigations related to allegations he misused taxpayer funds and punished staff for raising concerns about his spending.
Pruitt told the committee that some changes have been implemented since some of the scandals broke, including adding a new requirement that any spending over $5,000 needs to be signed off by several senior officials.
Udall also asked Pruitt about a tweet issued by EPA's official account last month mocking Democratic senators for failing to block the confirmation of Andrew Wheeler, a former coal lobbyist now working as EPA's No. 2 official.
The occasion would normally be a routine budget hearing for an agency head. Udall submitted for the record on an email released on Wednesday by Pruitt's former security chief Nino Perrotta, who left the agency weeks ago.
Republican Sen. Lisa Murkowski of Alaska, the subcommittee chairwoman, has told reporters that she was concerned about the allegations surrounding Pruitt, but that it was up to the Trump administration to decide his future.
"I am concerned that numerous important policy efforts that you are engaged in are being overshadowed because of a series of issues related to you and your management of the agency", she said.
For his part, Pruitt took the same approach as he did in a pair of contentious House hearings last month, largely steering clear in his opening statement of addressing the dozen probes he is facing on topics such as the installation of a $43,000 soundproof phone booth in his office, a cut-rate condo rental from a lobbyist previous year and his domestic and global travel expenses.
New UK East Coast rail franchise to be public-private partnership - minister
Stagecoach's chief executive Martin Griffiths estimated the company's total losses at £260m, in February. The RMT, the largest rail union, called for the switch back to the public sector to be made permanent.
A letter from the agency's inspector general also contradicted Pruitt and the EPA's claim that the need for a 24/7 security detail was a response to an assessment of death threats against him.
"As you know, the right answer is 'yes, '" Udall said.
The EPA's inside investigative workplace has accomplished investigations into 14 threats directed at Pruitt and his household, three of that are dated earlier than he was sworn in.
Pruitt has also faced growing pressure from some Republican senators who are not on the panel over his handling of USA biofuels policy.
Corn state senators like Iowa's Grassley have been infuriated by the EPA's decision to provide an unusually large number of waivers from the U.S. Renewable Fuel Standard (RFS) to refineries in recent months, freeing them from their obligation to blend biofuels like ethanol into the nation's fuel.
Environmental Protection Agency (EPA) head Scott Pruitt has been under scrutiny for his lavish travel expenses.
The study would show that the chemicals - known as PFOA and PFOS - are risky to humans in much smaller quantities than the Environmental Protection Agency has previously said, Politico reported this week.
In testimony before two House subcommittees last month, Pruitt sought to put the blame for any missteps on his subordinates, saying that his security team decided he should fly in first class and that his chief of staff approved the questionable raises.